Preface

Summary

The question addressed in this book is whether metaphysics is possible. If metaphysics is a science, why are we unable to make progress or reach unanimous agreements as we can with the other sciences? And if it is not a science, on what grounds do its claims to truth rest? At the moment, there is no standard for agreement on metaphysical questions, so there is no objective means for settling disagreements. As a result, all sorts of opinions are tossed about with no means of reaching definite conclusions.

The question of whether metaphysics is possible implies that the validity of metaphysics can be doubted. This implication may upset many readers: we don't like being told that a subject we have studied intensively might be useless. Nevertheless, Kant has become aware that metaphysics needs a sturdier foundation than it currenty has if it is to be taken seriously. He is confident that those who read his work carefully will agree.

Kant came to recognize the importance of finding a sturdy foundation for metaphysics when he read Hume, whom he claims roused him from a "dogmatic slumber." Hume inspired Kant by critiquing our concept of cause and effect, asking how we know that one event acts as a cause for another event. Hume concludes that we do not have a priori knowledge of causation: we cannot know the causal relationship between two events prior to our experience of it by means of reason alone. Instead, Hume suggests that what we call our "knowledge" of cause and effect is simply an expectation that one event will follow another based on habit rather than reason.

Hume's conclusion is fatal to metaphysics. If our "knowledge" of cause and effect is based on custom and habit rather than reason, then all the metaphysical theories that try to explain how our reason leads us to this knowledge are in vain. On further inspection, Kant found that all metaphysics is based on a priori reasoning, drawing connections between concepts without any reference to experience, so all metaphysics is potentially open to Hume's attack.

Kant explains how connections can be drawn a priori and how metaphysics is possible in his monumental Critique of Pure Reason. This book is long and difficult, however, and so he has written the Prolegomena to any Future Metaphysics as a shorter work that will make the ideas found in the Critique more accessible to a wider audience. The Critique of Pure Reason follows what Kant calls a "synthetical" style, deducing conclusions from first principles. The Prolegomena, on the other hand, follows an "analytical" style, breaking the problem down into simple bits and examining them individually.

Commentary

Metaphysics is the oldest and most respected branch of philosophy. It examines the constitution, nature, and structure of reality, and strives to uncover the underlying causes and foundations that make things the way they are. Physics simply describes the universe, and the laws of physics are only good for predicting what will happen. Metaphysics, by contrast, tries to explain the universe and why things happen the way they do. While physics is based on observation and experience, metaphysics is an a priori form of knowledge based on the unaided exercise of pure reason. Metaphysicians do no experiments: they try to sort everything out in their heads.

The nature of causation is an important topic in metaphysics. We can see in our day to day life that certain events seem to cause other events: one billiard ball may cause another billiard ball to move, or a fall from a great height may be the cause of a broken leg. The metaphysical question, then, is why and how one event can act as a cause for another. How do we know that a certain event is the cause of a later event, and not just a coincidental precedent? What is the nature of the causal connection between the two?

Hume's answer, in short, is that there is no discernable difference between two events that are related causally and two events that are just coincidentally conjoined. He argues that we say two events are causally connected if we see them frequently conjoined. Hume does not believe that we have a rational justification for doing so. We do not and cannot perceive the causal connection itself, and all our talk about cause and effect is based simply on the habit of seeing certain events happen one after another.

Kant notes that Hume's argument is an attack on the very possibility of doing metaphysics. Metaphysics tries to look behind the events themselves and see the fundamental connections and inner workings that tie things together. As a result, metaphysics relies on the assumption that the intellect has the power to see these fundamental connections and inner workings even if the senses do not. Hume's assertion that the intellect has no such power is thus a fatal blow to the very study of metaphysics.

Kant is willing to agree with Hume, but he is not as content as Hume is to simply conclude that metaphysics is misguided. Kant concludes instead that metaphysics is in need of clearer definition and stronger foundation if it is to be taken seriously. He complains that metaphysics is unscientific, that there are no standards for right and wrong, and that anybody's opinion is as good as anybody else's.

Kant's project, then, is to make metaphysics scientific. This means turning metaphysics into a systematic body of knowledge built on first principles. Newtonian physics, for instance, begins with Newton's three laws, which are based on careful observation and experience. Further physical principles are then deduced from these three laws. A new proposition can then be judged to be true or false quite easily based on whether or not it accords with the laws and principles that are already in place. Kant hopes to do the same for metaphysics so that disagreements and criticisms regarding metaphysical problems can be settled objectively, once and for all.

This project is part of what is called Kant's "critical" period. In his early career, he followed in the footsteps of rationalist metaphysicians such as Descartes, Leibniz, and Christian Wolff. The influence of Hume led Kant to write his three great "critiques": the Critique of Pure Reason, the Critique of Practical Reason, and the Critique of Judgment. These works, along with the Prolegomena, are "critiques" because they do not simply try to answer metaphysical questions, but ask instead how we know or how we claim to know the answers to these questions. Kant is primarily interested in knowing, for instance, how we can know that two events are connected causally, rather than what the nature of that causal connection is.

Preamble

Summary

Metaphysics consists of knowledge apprehended by pure reason. By definition, metaphysics studies what is beyond experience. The Greek root meta means "beyond," so "metaphysics" literally means "beyond physics." Like mathematics, metaphysics is an a priori body of knowledge.

The distinction between a priori and a posteriori ways of thinking is that the former are drawn from pure reason and the latter are drawn from experience. Kant goes on to draw a second, even more important, distinction between analytic and synthetic judgments.

The predicate of an analytic judgment is contained in the concept of the subject: the predicate, then, is simply an analysis of the subject concept. "All bachelors are unmarried" is analytic: being unmarried is a part of the concept of "bachelor," so saying that all bachelors are unmarried doesn't add anything to our concept of "bachelor"; it just clarifies the definition.

The predicate of a synthetic judgment, on the other hand, adds something new to the concept of the subject: it synthesizes two different cognitions. "All swans are white" is synthetic: we can know what a swan is without necessarily knowing that it's white, so learning that swans are white is an additional cognition that we can attach to our concept of "swan."

All analytic judgments are a priori, since they consist simply in the analysis of concepts and do not appeal to experience. Synthetic judgments, on the other hand, can be either a priori or a posteriori. Kant classifies synthetic judgments into three types: judgments from experience, mathematical judgments, and metaphysical judgments.

Judgments from experience are synthetic a posteriori since they are pieced together (synthetic) from the objects of experience (a posteriori).

Mathematics consists of synthetic a priori judgments. The concept of "7 + 5," Kant argues, contains the union of those two numbers in a single number, but the concept itself does not contain the number 12. We must make a leap of intuition in order to determine that twelve is indeed the number that results from the union of seven and 5. The same is true of geometry: the concept of the shortest distance between two points is not contained within the concept of a straight line. The temptation to think of math as analytic comes from the fact that the truths of mathematics are necessary: we cannot reasonably deny that 7 + 5 = 12. The fact of the matter is that mathematical cognitions require intuitive leaps that are synthetic in nature.

Metaphysics also consists of synthetic a priori judgments. It may seem that metaphysics consists largely of analytic judgments, since the only thing metaphysicians agree upon are the various definitions that are analytic in nature. However, metaphysics consists of synthetic judgments that are built upon these analytic definitions, much like mathematics consists of synthetic judgments built upon analytic axiomatic truths.

The need to ask whether metaphysics is even possible arises because there is little agreement over the synthetic judgments that ought to constitute it as a body of knowledge. Kant's proposed method is to start with the assumption that synthetic a priori judgments are possible, since they constitute both mathematics and pure natural science. He will investigate how synthetic a priori knowledge is possible in these fields in the hopes of discovering also how such knowledge might become a reliable source for metaphysics. He proposes to examine first mathematics, then pure natural science, and then ask how metaphysics is possible in general and as a science.

Commentary

The distinction between a priori and a posteriori shows the two possible sources of knowledge: the intellect and experience. If we can know something independently of experience, it is a priori, and if we know something through experience, it is a posteriori. Math is a paradigmatic example of a priori knowledge: I can figure out that 7 + 5 = 12 in my head, and nothing I find in experience can possibly contradict that knowledge. The statement "all bachelors are unmarried" is also a priori even though it refers to bachelors, which, unlike numbers, can be found in the world outside our heads. The reason is that "all bachelors are unmarried" is a definition of a bachelor rather than a statement based on experience. The statement, "all bachelors are lonely," on the other hand, is a posteriori, since loneliness is not a part of the concept of "bachelor." That statement is drawn from the speaker's experience with bachelors or from what other people have told him about bachelors.

While the a priori/a posteriori distinction is epistemological, distinguishing between sources of knowledge, the analytic/synthetic distinction deals with the logical structure of the judgments themselves. "All bachelors are unmarried" is analytic because the concept of bachelor is "unmarried man": this statement merely clarifies a part of the concept of "bachelor." A good test for whether a statement is analytic is to ask whether people could understand the subject concept if they did not know that the predicate were true of it. For instance, if I did not know that all bachelors are unmarried, I couldn't properly be said to understand what a bachelor is. On the other hand, "all swans are white" is synthetic because, even though we may generally think of white animals when we think of swans, I could be said to understand what a swan is without knowing that it is white.

Kant was the first person to draw the analytic/synthetic distinction explicitly. Until Kant, this distinction had generally been lumped together with the a priori/a posteriori distinction. Hume and others had considered the propositions of mathematics to be analytic. In proposing that there are synthetic a priori judgments, Kant suggests, contrary to conventional wisdom, that propositions like "7 + 5 = 12" are in fact synthetic. His argument is essentially that the concept of "7 + 5" is the union of the concepts of "7," "5," and addition. None of those three concepts in themselves contain the concept of "12"; it is a new concept that arises from the synthesis of the three subject concepts.

There are further arguments in Kant's favor. If concept of "12" were part of the concept of "7 + 5," then so would the concepts of "9 + 3" and "16 - 4," and an infinitude of other concepts. How could the concept of "7 + 5" possibly contain all these other concepts? Also, it seems ridiculous to suggest that the concept of "154,938" is a part of the concept of "52,624 + 102,314": we could understand the concept of that sum without necessarily knowing what the two numbers add to.

While analytic judgments consist simply of analyzing the subject of a proposition, synthetic judgments add something new to it. They effectively connect two independent pieces of knowledge to each other. Kant explores the nature of this connection. With synthetic a posteriori knowledge, the connection is made through experience. If I see a lot of white swans, I come to associate the concept of white with the concept of swan through experience. With synthetic a priori knowledge, the answer is more complicated. How do I learn to link the concept of "7 + 5" with the concept of "12"? In the sections that follow, Kant sets about trying to answer that question. His hope is that if he can explain how we can connect concepts in pure mathematics and pure natural science, he will also be able to explain how we can connect concepts in metaphysics.

The analytic/synthetic distinction is one of Kant's most significant contributions to philosophy. Like any significant contribution, it has been the subject of heated controversy ever since. One of the difficulties lies in saying precisely what the concept of "bachelor" or "swan" consists of. At what point can I no longer properly be said to understand what a thing is? If all humans are animals and all humans have noses, why is being an animal a part of the concept of being human and having a nose not? The idea that words have "concepts" attached to them goes all the way back to Aristotle's essences.

First Part

Summary

The first of the four questions Kant sets himself in the preamble is "how is pure mathematics possible?" If math consists of synthetic a priori cognitions, we must be able to draw connections between different concepts by means of some form of pure intuition. The word translated as "intuition" is the German word Anschauung, meaning literally a point of view or way of seeing. For Kant, intuition connects the two distinct concepts that are joined in synthetic judgments. Kant distinguishes between empirical intuitions and pure intuitions. Empirical intuition is what we normally call sense perception: in the synthetic proposition, "my cat has brown fur," my sense experience, or empirical intuition, leads me to connect the concept of "my cat" with the concept of "has brown fur" (this is not Kant's example).

Since math consists of synthetic a priori cognitions, there must be some form of pure intuition innate within us that allows us to connect different concepts without reference to sense experience. Kant's answer is that space and time are not things in themselves, to be found in the world, but are what he calls the "form of sensibility": they are innate intuitions that shape the way we perceive the world. Prior to any sense experience, we have no concept of the objects we find in space and time, but we still have the concepts of space and time themselves. Geometry is the a priori study of our pure intuition of space, and numbers come from the successive moments of our pure intuition of time. If space and time were things in themselves that we could only understand by reference to experience, geometry and math would not have the a priori certainty that makes them so reliable.

Neither space nor time, nor the objects we perceive in space and time, are things in themselves: the objects we perceive are mere appearances of things in themselves, and space and time are empty forms that determine how things appear to us. If space were actual and not built into of our mental framework, two things with all the same properties would be in every way identical. However, Kant points out, our left and right hands have all the same properties, but they are not identical: a left hand glove will not fit on a right hand. This suggests that space is not independent of the mind that perceives it.

These conclusions lead Kant to three final remarks. First, he points out that we can have a priori certainty of geometry, and thus of our understanding of spatial relations, only because we have a pure intuition of space. Our certainty comes because we are only examining our own mental framework, and not things in the world. Second, he responds to the potential accusation that he is engaging in idealism. Idealism claims that there are no objects in the world, only minds, and that everything we see is just a construction of the mind. Though Kant has argued that we cannot perceive things in themselves, but only appearances of things, he still maintains that things in themselves, independent of our perception, exist, and that they are the source of what we do perceive. Third, he points out that appearances cannot be deceptive. I can misinterpret what I see, and be deceived in this way, but I cannot be mistaken about the appearances themselves. If space and time were things in themselves, then we could misinterpret our perception of them and be deceived regarding them. However, since they are mere appearances, they are a priori certain.

Commentary

In the preface to the second edition of the Critique of Pure Reason, Kant claims his system has caused a "Copernican revolution in philosophy." The revolution he refers to is a reversal of our concept of space and time. Until Kant, it had been assumed that space and time were properties of the world, into which the objects of sensory experience were placed. Kant's radical reversal consists in claiming that space and time are not properties of the world but are rather properties of the perceiving mind. Space and time are like mental spreadsheets that organize how information is organized in our minds. Bertrand Russell explains this idea: "If you always wore blue spectacles, you would be sure of seeing everything blue. Similarly, since you always wear spatial spectacles in your mind, you are sure of always seeing everything in space."

Kant's argument for this position starts from the assumption that geometry and mathematics consist of synthetic a priori cognitions. To make synthetic judgments a priori, we must have some sort of pure intuition that allows us to draw concepts together without making any reference to experience. Geometry, for instance, gives us a priori knowledge about space, so our knowledge of space must be built into our minds. Therefore, Kant concludes, our concept of space is not something we learn from experience, but it is something we have prior to experience. Our concept of space is a feature of our minds and not a feature of reality. Kant believes he can make a similar argument about our concept of time with reference to our synthetic a priori knowledge of arithmetic.

If computers had existed in Kant's time, he would have had a useful metaphor for explaining the relationship between things in themselves, appearances, and our perceiving mind. We can compare things in themselves to data. Data in itself is invisible, and yet the programs we run are nothing more than data being interpreted. We can "read" data only once it's been through a processor and then projected onto a monitor. What we see on the monitor is not the data itself, but the "appearance" of the data. The processor and the monitor are like the pure intuitions of space and time: we cannot understand the thing in itself (data) until it has been made comprehensible by these intuitions. We do not perceive things in themselves but appearances of things. Our minds do not have the capacity to understand things in themselves just as we cannot understand data in itself by staring at a microchip.

Kant's discussion of space as an a priori form of intuition is meant to settle a debate between Newton and Leibniz regarding the nature of space. Newton maintains that space is absolute: it exists already as a thing in itself independently of the things that are in it. Leibniz holds to a relational theory of space, according to which space is a relational property that holds between objects. Space is not absolute, but dependent on the objects that are in it.

Both positions share the assumption that space is mind-independent. Though Leibniz does not, like Newton, believe that space is absolute, he believes that space only depends on the relations between objects and not on the minds that perceive space. Kant's example of how two internally identical hands cannot fit the same glove is meant to contradict Leibniz.

At the beginning of the twentieth century, with Einstein's theory of relativity, Kant's theory of space became the subject of heated controversy. According to Kant, our knowledge of space comes prior to experience, and Euclidean geometry can tell us everything we need to know about space. General relativity shows that the universe does not in fact conform to the laws of Euclidean geometry and that space and time are far more complicated than we think. Space and time, far from being pure intuitions that we can know a priori, are quite different from what our intuition tells us they are.

Kantians reply to this objection by saying that Kant is not talking about time and space in themselves but just about our cognition of time and space. Though space-time may be a four-dimensional curved space, our mind perceives space as flat, three-dimensional, and independent of time. Kant is not making a statement about how the world is, but about how the mind perceives the world.

The arguments for both sides are complicated, but over the course of the century, the defenders of Kant have diminished in number. The new physics of relativity and quantum mechanics have increasingly showed that any "intuitions" we might have about the nature or structure of reality are liable to be mistaken. Though Kantian constructions are arguably possible, this new physics makes a lot more sense if we assume that space-time exists independently of the mind.

Second Part, Sections 1426

Summary

The second part of the Prolegomena concerns itself with the question, "how is pure natural science possible?" "Natural science" is what nowadays we would simply call "science": it is the systematic body of knowledge that deals with nature. Kant remarks first of all that when we talk about nature we are not talking about things in themselves, which, as he has already claimed, we can know nothing about. Rather, we are talking about the objects of experience as they appear to us. For our study of nature to be a science, these experiences must conform to universal and necessary laws. Kant observes that we do indeed study natural science and make use of universal and necessary laws. There is some kind of pattern or regularity in our experience, but how is this possible?

Kant draws a distinction between judgments of perception and judgments of experience. Judgments of perception bring together several empirical intuitions and are only subjectively valid. For instance, I may see the sun shining brightly and feel that a rock under the sun's rays is warm, and judge that the rock grows warm under the sun. This judgment draws together the intuitions that the sun is shining and the rock is warm, but it is still valid only for me and only at that particular time.

Judgments of experience apply pure concepts of the understanding to judgments of perception, turning them into objective, universally valid laws. For instance, I can apply the concept of cause to my earlier judgment that the rock grows warm under the sun and judge that the sun caused the rock to grow warm. We do not find pure concepts of the understanding in experience. Rather, they are concepts we use to structure our understanding of experience. They are a priori concepts we use to draw together and make sense of our various judgments of perception. Because these concepts are a priori, they are also universal and necessary. Thus, judgments of experience are the synthetic a priori laws that make natural science possible.

Essentially, the distinction is that judgments of perception deal only with what we sense, or intuit, while judgments of experience deal with what we infer from our perceptions. We cannot dispute judgments of perception because they are wholly subjective: you cannot tell me the car didn't seem red to me. We can dispute judgments of experience because they are meant to be objective: you can tell me the car wasn't red.

Section twenty-one categorizes the different kinds of judgments, concepts of the understanding, and universal principles of natural science into three separate tables. These tables are reproduced in a special section in this SparkNote entitled "Kant's Tables of Categories."

The table of judgments divides judgments into their logical parts. Every judgment must have one of the three kinds of quantity, quality, relation, and modality. For instance, the judgment "the sky is blue" is singular (it deals with the sky), affirmative (it affirms that the sky is blue), categorical (it is a simple subject-predicate sentence), and assertoric (it makes an assertion).

The table of the concepts of the understanding lists the concepts that correspond to the logical parts of judgments. By applying a concept to the corresponding judgment, we can turn a judgment of perception into a judgment of experience. For instance, the concept that corresponds to an assertoric judgment is "existence," so we can make the objective judgment that a blue sky exists.

The table of universal principles classifies four different kinds of law that correspond to the four different kinds of concepts of the understanding.

Commentary

Kant is complicated, but there is a very clear structure hiding underneath all this difficult vocabulary. Essentially, Kant is building a complex system to explain how we make sense of the world.

At a very basic level, Kant distinguishes between things in themselves, and our perceiving mind. The first question is how can we perceive things in themselves? How does our mind make contact with anything outside it? Kant answers that we cannot perceive things in themselves directly; all we can perceive are sensations, the impression things in themselves make upon our senses.

Our mind perceives sensations, but must impose some sort of form on these sensations for them to be intelligible. This form is our intuition of space and time. By subjecting the sensations we perceive to the intuitions of space and time, we get empirical intuitions. Empirical intuitions are what we might refer to as "sense-data": they are what I see, hear, or feel at any given moment.

If all I had were empirical intuitions, life would be a meaningless blur of unintelligible sensations. In order to make sense of experience, we must first draw connections between empirical intuitions. Judgments of perception join two or more intuitions, associating them with one another. Seeing the bright, shining sun and feeling the warm rock are two separate empirical intuitions: a judgment of perception makes a connection between the two.

Judgments of perception are subjective. I can draw a connection between the sun and the warm rock, but I can't relate that connection to any of my past or future experience, and I can't relate it to anyone else's experience. Empirical intuitions and judgments of perception come from our faculty of sensibility, which deals with our senses and what they tell us. To give objectivity or universality to our experience, we must subject it to our faculty of understanding, which deals with our capacity for thought and concept formation.

Kant infers that we must use concepts of pure understanding to turn judgments of perception into judgments of experience because empirical intuitions in themselves cannot be generalized. Judgments of perception are particular and subjective: only a priori concepts can be universal and objective. As Hume was right to observe, we cannot find universal concepts like "every event is caused" in experience. Kant concludes that such concepts are a part of the understanding: we do not find them in experience; we apply them to experience.

Kant has a two-step schema that explains how we come to see the world. In the first step, which deals with our faculty of sensibility, we have things in themselves providing sensations that are then given subjective form by our pure intuitions of space and time. Sensations combined with pure intuitions make empirical intuitions. In the second step, which deals with our faculty of understanding, these empirical intuitions are given objective form by the pure concepts of the understanding. Empirical intuitions combined with pure concepts of the understanding make the appearances that constitute experience.

We should not mistake Kant's system for elaborate psychology. He is not giving a map of the human mind, or explaining how it is that we come to cognize things. Rather, he is examining what we find in experience, and analyzing its parts. His procedure is logical rather than psychological. He recognizes, for instance, that we have a concept of cause and effect, but that that concept cannot possibly be derived from experience. Thus, he concludes that we must have some faculty that leads us to see the world in terms of cause and effect. Similarly, he argues that our understanding of time and space cannot itself be found in experience, and so must also rely on our intuition.

Ultimately, mere sensations constitute very little of what we consider to be our experience of the world. A great deal of our experience comes from our inner faculties. Though none of these faculties can actually "say" anything themselves, they give shape and form to our sensations, and thus deeply influence how we experience them.

Second Part, Sections 2739

Summary

Hume's skepticism arises when he asks how we perceive causal connections between events. Reason alone cannot tell us about connections between things in the world, and experience alone cannot infer universal generalizations such as "every event has a cause." Hume concludes that in fact we have no rationally justified knowledge of cause and effect. He suggests instead that our concept of causation is justified only by the habit of seeing certain events follow from certain other events.

Kant agrees that we cannot discover the concept of cause and effect either in experience or by means of reason. However, he does not conclude with Hume that this concept is merely a result of habit or custom.

Rather, he suggests, causation is an a priori concept of the understanding applied to appearances. We can know nothing about things in themselves; we can know only how they appear to us in the form given to them by our faculties of sensibility and understanding. The concept of cause and effect is not to be found in these appearances; rather, it is part of the form given to them by the understanding. Causation is not a "thing" that we can discover, either by means of reason or experience. Causation is a form given to experience that makes it intelligible to us. Hume asked how we can derive pure concepts (such as causation) from experience, and answered that we cannot. Kant agrees: we cannot derive pure concepts from experience; rather, we derive experience from these pure concepts.

Pure concepts of the understanding make experience legible, so to speak, but cannot tell us anything about things in themselves. Because pure concepts, as well as our pure intuitions of space and time, are a priori and therefore necessary, we are tempted to think that they can give us knowledge beyond that which we find in experience. However, our pure concepts and pure intuitions provide only form, and no content. They help us make connections between appearances, and as such, they deal only on the level of appearances. They cannot tell us anything about the things in themselves behind these appearances.

Nature, understood as the totality of all our sensations, is possibleas we saw in the first partby means of our pure intuitions of space and time. Nature, understood as the totality of experience as understood and connected by laws, is possibleas we saw in this part by means of our pure concepts of the understanding. We cannot go farther and ask how the faculties that give us our pure intuitions and pure concepts are possible, because it is precisely these faculties that help us make sense of experience. We have no further faculty that would help us understand what is behind these faculties.

Sensations themselves teach us nothing about the connections between them or the laws that govern them: these are all provided by our faculties of sensibility and understanding. These faculties, then, are what make nature itself possible, insofar as nature is our intelligible experience. Whatever laws or universality we find in experience comes not from the sensations themselves but from the form given to them by our faculties. Thus, Kant concludes: "the understanding does not derive its laws (a priori) from, but prescribes them to, nature."

Kant wraps up this part with an example showing how we derive astronomical principles from our pure intuition and concepts rather than from experience itself, and with an appendix discussing his system of categories.

Commentary

Early modern philosophy, from Descartes to Hume, is roughly divided between rationalistslike Descartes, Spinoza, and Leibnizand empiricistslike Locke, Berkeley, and Hume. Rationalists place a strong emphasis on pure reason and metaphysics, suggesting that the unaided intellect can discover metaphysical truths about the nature of life and the universe. Empiricists place a strong emphasis on knowledge gained from experience, confining the intellect to reasoning about experience based on what it finds in experience. Kant attempts a synthesis between these two camps by showing that they are both flawed in a fundamental respect. The empiricists are wrong to think of experience as consisting only of sensations, and the rationalists are wrong to think that the intellect can give us any insight into the essence or nature of things in themselves. Kant attacks both empiricists and rationalists in this part of the text.

His main object of criticism in the empiricist camp is Hume, who Kant nonetheless deeply admiresafter all, Kant's system was motivated by Hume's skepticism. Hume's mistake, according to Kant, lies in misunderstanding the nature of experience. Like pretty much everyone before Kant, Hume believes that experience is fundamentally a set of simple impressions. Ideas and complex impressions are built by connecting various simple impressions. Experience for Hume is what Kant would call "sensations": it is the simple sense-data of what we see, hear, smell, etc.

Kant criticizes this view by pointing out that experience is more than just simple impressions that we receive as neutral observers. Experience comes in an already organized form. Everything that we can experience happens in space and time. Space and time, Kant argues, are not impressions or sensations: we do not learn about them through experience because they are pure intuitions. Further, everything that happens in space and time is subject to the law of cause and effect. This law is also not an impression or a sensation but a pure concept of the understanding. Pure intuitions and concepts organize experience for us and give it its form. We are not neutral observers of an objective world; we actively shape the world we perceive so as to make it intelligible.

Hume argues that we can have no knowledge of cause and effect because we cannot find causes or effects in the simple impressions that constitute the entirety of our experience. Kant agrees that there are no causes and effects to be found in experience, but suggests that our understanding applies the concept of causation to experience in order to make it intelligible. To modify Bertrand Russell's analogy, we see the world through causation-colored sunglasses.

If we can dismiss Hume's skeptical empiricism by pointing out that we can have a priori knowledge of cause and effect, space and time, we encounter an equally unsavory dogmatism in the rationalist camp. If synthetic a priori knowledge is possible, then we can learn substantial truths about the universe that have a necessity and universality that we cannot find in experience. While experience can only tell us about appearances, about how things seem to us through the imperfect medium of the senses, reason can tell us about things in themselves.

Kant's answer to this temptation is twofold. First, our a priori pure intuitions and pure concepts only help us to make sense of appearances. Kant makes no claim that the laws of cause and effect apply to things in themselves, only that we apply them with necessity to appearances. Second, our pure intuitions and pure concepts do not give us any substantial knowledge. All they do is prescribe the form our experience takes. They are the cup into which experience is poured, so to speak: we cannot drink the cup, but without the cup we wouldn't be able to drink at all.

Kant's pure intuitions and pure concepts reconcile the empiricist and rationalist camps by showing that, on one hand, we can have a priori knowledge relating to experience and on the other hand, this a priori knowledge does not tell us anything about things in themselves. Kant recognizes that we are not passive recipients of our sensory experience. What we perceive is given its form by faculties that are innate. These faculties do not tell us anything about what the world is really like, but they determine the patterns according to which the world appears to us.

Third Part, Sections 4049

Summary

The Third Part deals with the question, "How is metaphysics in general possible?" We have seen how both mathematics and pure natural science are possible, by appealing to our pure intuitions of time and space and the concepts of our faculty of understanding. We use our pure intuitions and our faculty of understanding to make sense of experience, but metaphysics, as its name suggests, deals with matters that are beyond the realm of experience. It either deals with concepts that lie outside of experience (like God) or it deals with the totality of possible experience (like whether the world has a beginning and an end). Intuition and understanding are of no use here. Metaphysics deals with the faculty of pure reason, and the ideas contained therein.

The distinction between the understanding and reason is crucial. Philosophical error frequently arises from a confusion of one for the other. Any concept that can be applied to experience belongs to the faculty of understanding and has nothing to do with metaphysics. Reason is not directed toward experience, and any attempt to apply the ideas of reason to experience is mistaken.

Reason tries to make experience complete. Reason tries to tie all of experience together and to give it meaning. This drive to metaphysics is not in itself problematic; it becomes wrong-headed only when we apply our pure intuitions or pure concepts of the understanding to the pursuit.

Kant distinguishes three different kinds of "ideas of reason" psychological ideas, cosmological ideas, and the theological ideathat between them contain all of metaphysics. This summary will deal with psychological ideas, while the summary of sections 5056 will deal with cosmological and theological ideas.

Psychological ideas try to identify some sort of substance or ultimate subject underlying all the predicates we can apply to a subject. For instance, we can describe a cat as "a thing with claws" or "a thing that purrs" and so on, but what is the "thing" itself? What do we have left over when we peel away all the predicates? Kant suggests that this search is futile: the understanding helps us make sense of experience by applying pure concepts to empirical intuitions, and concepts take the form of predicates. The only knowledge we can have comes in the form of predicates attached to subjects.

A possible candidate for ultimate subject comes in the form of the thinking ego, or soul. When describing internal states ("I think," or "I dream," for example), we refer back to an "I" that is fundamental, indivisible, and unique. However, Kant argues, this "I" is not a thing or a concept that we can have knowledge of in itself. That we are capable of experience at all suggests that we have some sort of consciousness, but we refer to this consciousness (or soul) without having any substantial knowledge of it.

Just as appearances in the external world suggest to us that there are things in themselves, so inner sensations suggest to us that we have some sort of soul or ego. But, just like things in themselves, we can know nothing about this soul; we can know only about the appearances that manifest themselves. This conclusion suggests that Descartes is wrong in thinking we can know about the mind better than we can know about external bodies.

What we can say about the soul we can say only in reference to our own experience. Thus, we can't know whether or not the soul is immortal, since that is to ask questions about the soul outside the realm of experience.

Commentary

In the Prolegomena, Kant divides mental activity into three major faculties. First, there is the faculty of sensibility that uses the pure intuitions of space and time to form our sensations into empirical intuitions. This faculty helps us organize and make sense of what we see, hear, smell, touch, and taste. We can also use the pure intuitions of space and time to reason mathematically.

Second, there is the faculty of understanding that uses pure concepts to form our empirical intuitions into appearances. This faculty allows to make sense of what we see, hear, smell, touch, and taste as according itself with regular, universal laws of nature, and so helps us make general inferences and conclusions. That, effectively, is the business of natural science

Third, Kant introduces us to the faculty of reason in this part of the text. While the faculties of sensibility and understanding help us make sense of experience, reason helps us make sense of purely mental concepts. It does this by means of ideas, which try to fill out and give completeness to the concepts we apply in experience. For instance, psychological ideas take the concept of substance and try to flesh out what it is we mean when we talk about a "thing." What substance underlies a thing and makes it what it is? We will see similarly that the cosmological ideas try to fill out our concept of cause, trying to identify connections that go beyond those we encounter in experience, and that the theological idea tries to fill out our concept of community, trying to identify what unifies everything that exists.

Our faculty of sensibility gives us math, our faculty of understanding gives us science, and our faculty of reason gives us metaphysics. The important conclusion to draw from this discussion of faculties is that metaphysics is the product of pure reason and deals only with ideas in our head; in other words, metaphysics cannot tell us about how things are in themselves. Metaphysics as Kant conceives it is more a matter of untying mental knots than determining whether or not God exists.

The mental knot Kant associates with psychological ideas is that of substance, and particularly that of a thinking substance. Talk of substances was a major preoccupation of the rationalist metaphysics of the 17th and 18th centuries, and Descartes was one of the major philosophers to discuss substances. Descartes is famous for the statement "I think therefore I am": I cannot doubt that I exist, since the act of doubting is an act of thought and I couldn't think unless I existed. I exist: but what can I know about this "I" that I am? While I know that I think, I can doubt that I have a body (I could be a butterfly dreaming I have this body), so I conclude that I am a thinking (as opposed to bodily) substance. I may think I know, or I may guess, any number of things about my body, but while these thoughts or guesses may be mistaken, I cannot doubt that I am thinking or guessing. From this line of reasoning, I conclude that my mind is better known to me than my body.

And so on. In the Meditations, Descartes questions the reliability of the senses, and then attempts to see how much he can know about himself and the world around him using only his intellect.

According to Kant, all I can know about this "I" is that I am. What I sense and think are representations, and these representations have to take place within a subject. For things to be seen and heard there has to be a consciousness that is doing the seeing and hearing. This "I" essentially represents that logical necessity: there must be something doing the seeing and hearing, and I call that something "I."

This "I" is not something I encounter in experience; it is the basis for my experience. As a result, we cannot apply to it the categories we apply to experience. Descartes tries to do essentially that, applying the concept of substance and other concepts of the pure understanding to it. Kant suggests on the contrary that we should think of this "I" the way we think of things in themselves: we can infer that it is, but we cannot infer anything about it. Pure reason, engaging in metaphysics, cannot tell us anything substantial about the way things are.

< Previous SectionThird Part, Sections 4049Next Section >Conclusion

Third Part, Sections 5056

Summary

Kant expresses the cosmological ideas as four distinct antinomies, or pairs of seemingly contradictory metaphysical propositions. They are:

(1) The claim that the world has a definite beginning and end vs. the claim that the world is infinite

(2) The claim that all things are made up of simple, indestructible, indivisible parts vs. the claim that everything is composite and infinitely divisible

(3) The claim that we can act in accordance with our own free will vs. the claim that everything we do is determined by nature

(4) The claim that there are necessary causes vs. the claim that nothing is necessary and everything is contingent

None of these claims can be verified in experience, and so we are tempted to think that they deal not with appearances but with things in themselves. Reason by itself seems capable of proving either side of each antinomy. Rather than come down on one side or the other, Kant proceeds to show how each antinomy results from a misunderstanding of the matter being discussed.

The mistake in the case of (1) comes from treating space and time as things in themselves rather than as intuitions of our faculty of sensibility. Space and time are features of our experience, and do not exist independently of experience. It makes no sense to ask whether or not the world has a limit in space and time, since that limit would exist outside the realm of our experience.

In (2), when we talk about the parts into which a composite thing can be divided, we are assuming that these already exist, waiting inside the composite thing. But these parts are only appearances, and so cannot have any existence until they are experienced.

In (3), causal necessity and freedom are made to seem contradictory when in fact they are compatible. The laws of nature can only operate within the confines of space and time, and so are applicable only to appearances. Freedom, on the other hand, is the ability to outside the confines of causality, and so to exist outside the confines of experience. Freedom, then, is applicable only to things in themselves.

Our faculty of reason does not deal with experience, and so we are free in our capacity as rational beings. This freedom must express itself only in general maxims that do not depend on causal influence or particular times and places. In obeying these general maxims, we still follow regular laws in the world of appearances. Thus, we can be free and also be subject to the laws of nature.

The seeming contradiction in (4) is similarly resolved if we see that one half of the proposition talks about things in themselves and the other half of the proposition talks about appearances. In the world of appearances, every causal connection may be contingent, which is to say it could have happened otherwise. Nonetheless, these appearances might have a necessary connection to things in themselves.

Kant deals very briefly with the idea of a God. In the Critique of Pure Reason, he shows at length the flaws in all the supposed proofs for the existence of God. Here, he simply points out that any "proof" of God's existence is a purely intellectual exercise, and cannot lead us to fundamental and substantial conclusions regarding the nature of experience.

In conclusion, Kant remarks that while there are justifiably many mysteries regarding what we find in experience, there should be no insoluble problems in the realm of pure reason. These problems deal only with reason itself and do not reach beyond our own minds into experience.

Commentary

The four antinomies Kant presents as "cosmological ideas" are common topics of metaphysical debate. In each case, Kant applies his distinction between appearances and things in themselves in order to resolve the antinomy. In the first two, he shows that both sides of the antinomy mistake appearances for things in themselves, and concludes they are both false. In the second two, he shows that two seemingly contradictory points of view are actually both acceptable so long as we recognize that one is applied to appearances and one is applied to things in themselves.

The first antinomy assumes that space and time exist independently of our experience and asks whether or not they have any limits. The second antinomy assumes that the objects of our experience have an independent existence and asks whether or not they have fundamental, simple parts. In both cases, we are trying to extend our knowledge of phenomena we've experienced beyond our experience of them. Kant reminds us that the objects of experience are mere appearances, and that the space and time in which we perceive them are constructs of our pure intuition. In other words, they do not exist beyond our experience of them.

Both these antinomies might seem a little odd in the light of modern physics. We have found a limit to space and time in the Big Bang, and we have identified the simple parts of objects in atoms and the elementary particles that constitute these atoms. Still, Kant could point out that these discoveries have been made in the realm of physics, not metaphysics. What we have discovered are the limits of observable experience, not the limits of things in themselves. The things in themselves that are the source of these appearances exist outside the realm of space-time and scientific observation.

The third antinomy is probably the most interesting, as Kant's answer to it is his ethical theory in a nutshell. The problem of free will is an old one, and a favorite topic of philosophical debate. If we had no free will, we couldn't be held responsible for what we do: we would be able to excuse our wrongdoings by saying "I had no choice." Freedom, then, consists in having a choice, in one's actions not being predetermined by outside forces. However, nature's laws dictate that every event is caused by some previous event, and that every event in turn acts as a cause for some subsequent event. How can we be said to have free will or to act independently of outside forces, without violating these laws?

Kant's answer is that cause and effect are products of the faculty of understanding and can be applied only to appearances, while freedom is a product of the faculty of reason and has nothing to do with appearances. Because freedom has nothing to do with appearances, it is outside the boundaries of time and space. As a result, a free act cannot be contingent on the particularities of what is happening at a particular time or in a particular place. Free acts must abide by general maxims. This theory is more fully explained in Kant's Groundwork for the Metaphysics of Morals, in which he claims that free acts take the form of a "categorical imperative" which insists that our actions follow maxims that we could will as general laws. Freedom does not mean spontaneity; it means obeying our own law. Because our freedom manifests itself in an orderly, law-like manner, it does not violate the laws of nature that apply to all appearances.

The fourth antinomy deals with necessity and contingency. The question is whether things necessarily happen the way they happen or whether they might have happened otherwise. To reconcile this antinomy, Kant identifies two different kinds of causation: a contingent one that determines how causes work in the world of appearances, and a necessary one that determines how things in themselves cause the appearances we experience.

Kant talks here about things in themselves as acting as causes and as being necessary, but both necessity and cause are pure concepts of the understanding, and are thus applicable only to appearances. Kant could excuse himself by saying he does not use terms like "cause" and "necessary" literally, but simply for lack of a better expression. Language can only describe the world of appearances, and when dealing with things in themselves it is inadequate.

Kant's discussion of the idea of God is very brief, mostly because his reasoning doesn't change: ideas of reason can only help us sort things out in our heads, but they can't tell us anything substantial about the world outside our heads.

Conclusion

Summary

In the Third Part, Kant discusses the various ideas of reason, and how they mislead the understanding into posing insoluble metaphysical questions. In this section, he hopes to determine the value of reason and the precise bounds within which it can operate. While we can never know more about an object than what experience teaches us, the concepts of our understanding help us to pose metaphysical questions that experience cannot answer. It is only natural, then, that we should consult reason when experience lets us down.

Kant distinguishes metaphysics from mathematics and science by saying that the former has bounds while the latter two have only limits. Both math and science are complete in and of themselves: there are no insoluble problems in these fields, no questions that cannot be answered given enough time, insight, and progress. They are limited only in that their scope is not absolutely general. Math cannot answer metaphysical or moral questions, and science cannot give us insight into things in themselves. However, morality and metaphysics are not needed in mathematical explanations and the nature of things in themselves does not affect the progress of science, which deals only with the objects of experience. What these fields don't know can't hurt them.

Metaphysics, on the other hand, is bounded: reason poses questions for itself that it cannot answer. In investigating metaphysical questions, reason bumps up against boundaries that it cannot press beyond. That is, metaphysics asks questions about the nature of things in themselves, but we cannot gain definite knowledge of anything outside experience.

However, these bounds can be useful. While we cannot know what is beyond them, we can infer from the existence of these bounds that there is something beyond them (i.e. things in themselves) and we can infer the connection these things in themselves must have with the perceptible world. While we cannot reach beyond experience to things in themselves, we can examine the relation between things in themselves and our experience.

Kant has already dismissed any attempt to prove the existence of God or to learn anything positive about God's nature. Our knowledge is structured by categories and concepts that are applicable only to experience, so we cannot apply these categories and concepts in any meaningful way to things beyond experience. For instance, it would be a mistake to attribute supreme rational powers to a Supreme Being, since we can't attribute anything to something beyond experience. What we can do, however, is attribute the rational order of the experienced world to a Supreme Being that sits outside the experienced world. This is not to say anything about a Supreme Being, but only about the relation that Being has with the world. If we see the world as structured in a rational way, we find a unity in experience by stretching our powers of reason right to the bounds of experience.

Though there is no way of knowing the reason why we have reason, Kant offers some speculation. He suggests that perhaps reason, in showing us the bounds of experience, also teaches us that there is something beyond experience that we cannot know, thereby giving us a more balanced perspective. Without the idea of a soul, we might think psychology can fully explain human behavior; without the cosmological ideas, we might think nature is sufficient unto itself; without the idea of God, we might become fatalists, doubting the possibility of free will.

Commentary

In the Third Part, Kant dismisses standard metaphysical questions and debates as pointless. He argues that these questions arise from a failure to distinguish between appearances and things in themselves, and from trying to apply the concepts of the understanding to something other than the objects of experience. In this conclusion, he shows us that reason and metaphysics do in fact have a very important valuejust not the value we normally think of them as having.

The distinction between math and science on the one hand, and metaphysics on the other, rests on the important distinction Kant makes between limits and bounds. Math and science are limited, meaning that they cannot tell us everything. No mathematical equation can tell me whether cloning is wrong, and no scientific experiment can tell me whether or not God exists. (These limits, incidentally, become all the more important in an age when scientific advances pose increasingly complicated moral dilemmas.) However, there is no boundary to what math or science can solve within their particular realms. This is not to say we have solved or can solve all the problems in these fields, but just to say that there is no external constraint on what can be learned. There may be puzzles we will never solve, but they will nonetheless have solutionsjust solutions we are incapable of finding. Neither math nor science will ever confront us with a puzzle to which there is no solution.

While math and science are limited, metaphysics is bounded. That means that there are metaphysical puzzles to which there is no solution. The ideas of reason deal with precisely that. "What is the nature of the soul?" is a question, says Kant, to which we cannot give an answernot because we do not know the answer, but because there is no answer to give. Metaphysics tries to deal with things in themselves, but all our concepts and intuitions are suited only to dealing with appearances. Our reason poses riddles for us for which there is no answer. Metaphysics is an attempt to reach for things that are beyond our grasp.

If metaphysics is bounded in such a way that we can never answer any of the questions it poses, we might think of it as a useless discipline. Kant suggests, on the contrary, that its value lies precisely in the establishment of these bounds. We cannot know what is beyond experience, but in reaching for it, we know there is something beyond experience. If something is bounded, that suggests there is something outside those bounds. Limits do not teach us this.

If all we had were our faculties of sensibility and understanding without reason, and all we had was math and science without metaphysics, we would have no awareness of things in themselves. We would pursue math and science with the assumption that we were learning everything there is to learn. We would assume that the concepts of science can explain all natural phenomena, and that whatever science can't explain doesn't exist. For instance, we would assume that a mind is nothing more than a brain, and that thought is nothing more than the firing of neurons.

Though reason cannot teach us anything about things in themselves, it gives us an awareness that there are things in themselves, and thus gives us an awareness of the bounds placed upon our learning. Metaphysics is important to us precisely because it is bounded. It gives us perspective, something that none of the more complete sciences can give us. While math and science teach us what we can know, metaphysics teaches us what we can't know.

Solution

Summary

The metaphysics of previous generations, which Kant discusses in the Third Part, is dialectical nonsense: debates as to the nature of the soul or the possibility of freedom can go on and on and back and forth forever without reaching any satisfying conclusions. He cannot deny that metaphysics exists as a disposition of human reason (we are naturally drawn to metaphysical questions), but he does deny that metaphysics as it has been conducted can lead to any real knowledge.

Kant is now finally ready to answer what he posed as the general question of this book: "How is metaphysics possible as a science?" His answer, effectively, is one word: "critique." Our faculty of reason cannot teach us anything about what lies beyond experience or about things in themselves, but it can help us to categorize and classify the various concepts of our faculties of sensibility, understanding, and reason. Rather than use reason to look outward, we should turn it inward and direct it toward itself.

Kant considers science to be a body of synthetic a priori knowledge. The faculty of reason has no power to gain a priori knowledge of things outside experience, or even outside the intellect. What it can do is survey the mind in its entirety and gain knowledge a priori about the nature and variety of our many concepts and faculties.

Kant is confident that anyone who has read the Prolegomena will conclude with him that nothing to date has advanced metaphysics in the slightest, and that metaphysics as it has been conducted to date is useless. Nevertheless, Kant remarks, we are naturally drawn to metaphysics and cannot just abandon it. For this reason, he expects formerly dogmatic metaphysicians to begin advancing the critical philosophy he envisions with great vigor.

Kant challenges anyone who disagrees with his dismissal of dogmatic metaphysics to provide one example of a metaphysical synthetic a priori judgment that has been proved with certainty. Such a judgment cannot be based on probability or conjecture, since a priori truths are necessary, and it cannot be based on common sense, since we derive common sense from experience. While common sense is useful for practical purposes, it cannot advance metaphysics as a science.

Commentary

Kant uses the term "metaphysics" to talk about two very different things. On one hand, he talks about the "dogmatic" metaphysics that he attributes to his rationalist predecessors, and on the other hand, he talks about the critical metaphysics he intends to set up in its place. In examining this distinction, we should also get a clearer sense of what a "critique" is, and why Kant thinks "dogmatic" metaphysics is not a science but that his critical metaphysics is.

"Dogmatic" metaphysics pursues the kinds of questions outlined in the Third Part as prompted by ideas of reason. These questions ask about the nature of the soul, the possibility of freedom, the ultimate constituents of matter, the existence of God, and so on. Metaphysics relies entirely on the faculty of reason, and Kant tries to show us that reason cannot get us any closer to answering these questions. The faculty of reason cannot connect with anything outside the mind, and certainly not with things in themselves.

If we recall, a science is a body of synthetic a priori knowledge. That is, it is a field of study that makes interesting, non- analytic judgments, but does so without any reference to experience. In order to make synthetic judgments without reference to experience, our mental faculties must be able to make significant connections within their own pure concepts. Our faculty of sensibility can use its pure intuitions of space and time to make mathematics and geometry. Our faculty of understanding can use its pure concepts to make natural science possible. Our faculty of reason has ideas, so the pressing question is what kind of synthetic judgments can these ideas produce?

We have seen that the ideas of reason pose all sorts of metaphysical questions that reason cannot answer. We have also seen that in doing so, reason pushes itself to the bounds of human knowledge, giving a sense of completeness and unity as to what we can know. Reason, then, has a sense of what kind of knowledge is possible, and so is ideally suited to examining the different mental faculties and determining precisely how knowledge is structured. The Prolegomena itself has essentially employed this technique: throughout, Kant has been investigating the different kinds of knowledge we have and the grounds on which this knowledge is justified. His conclusions that there are three mental faculties (sensibility, understanding, and reason), that the faculty of sensibility contains pure intuitions of time and space, or that the faculty of understanding is structured according to the concepts listed in his table of categories, are all conclusions reached through a critical investigation of the structure of knowledge.

While "dogmatic" metaphysics asks what we can know, Kant's critical metaphysics asks how we can know. A "critique" is an investigation that looks inward rather than outward, that investigates knowledge itself rather than the objects of knowledge. The Prolegomena is a shortened version of Kant's great work, the Critique of Pure Reason, which is an attempt to investigate how and what our faculty of reason is capable of.

Kant is not doing psychology. He is not trying to figure out how the mind works or anything like that. Rather, he is trying to figure out how knowledge works, and any claims he makes about the workings of the mind are based on his conclusions as to how knowledge must be structured in the mind.

One of the most significant conclusions of Kant's critical philosophy is that many concepts we think of as objectivelike space, time, or causationare in fact part of the way we structure knowledge. These concepts, as Kant shows in the Third Part, are often the source of perplexing metaphysical conundrums. In showing that these concepts are not to be found in the world, but rather in our own faculties, Kant is essentially redirecting metaphysics. He is telling us that we should not apply metaphysical concepts to the world but to our own faculties. All metaphysics can do for us is tell us how we know what we know. It cannot tell us what we cannot know.

Appendices

Summary

Kant takes it as self-evident that it is in everyone's interests to establish metaphysics as a science that proceeds according to agreed-upon and well- grounded principles. Kant's work proposes to do just that, so he feels he is only open to criticism if he has failed to do so, or if he is mistaken in claiming that metaphysics until now has been unproductive. If someone wishes to make the latter accusation, he challenges them to show him one metaphysical truth that is grounded with certainty and agreed upon by all.

The second, and longest, appendix deals primarily with an unfavorable review of Kant's Critique of Pure Reason published in a journal of metaphysics. The reviewer dismisses Kant's work as unclear and unoriginal metaphysical idealism. He makes no mention of Kant's central project of trying to make metaphysics scientific, and does not once refer to Kant's important category of the synthetic a priori. Though this reviewer clearly misunderstood Kant entirely, Kant takes this opportunity to clarify what he means by "transcendental" or "critical" idealism.

Traditional idealists, like Berkeley, assert that all the objects of experience are illusory, and that only pure concepts contain truth. Berkeley claims we have only a posteriori knowledge of space and all the objects in it, and that we therefore cannot be certain of this knowledge. Kant, on the other hand, asserts that we can know about space and time a priori, and that we can have certainty with regard to our experience. Contrary to Berkeley, Kant asserts that our pure concepts are illusory and that only experience contains truth.

Kant's motive for writing his Critique of Pure Reason was that there is no standard for agreement on judgments in metaphysics. This being the case, he does not feel his reviewer's judgment has any solid basis. He challenges his reviewer to produce one metaphysical synthetic a priori principle that can be used to contradict what Kant has written. Better yet, he offers his reviewer the choice of any one of the eight metaphysical propositions listed in his discussion of cosmological ideas. Kant wagers that he could provide a "proof" of the contrary position that his reviewer would not be able to dislodge. He suggests further that he could then provide a similar "proof" of the reviewer's own position, in order to show what dire straits metaphysics stands in.

If people acknowledge the problems in contemporary metaphysics, and study his work as a possible alternative, Kant is confident that much headway can be made, not only in metaphysics, but in other fields as well. For instance, it could free theology from dogmatism and speculation, as well as from the shallow mysticism that masks itself with dogmatic metaphysics.

Commentary

Kant's Critique of Pure Reason was met mostly with bewilderment when it was first published in 1781. The Prolegomena, published in 1783, was primarily intended to clarify and simplify what was said in the Critique in order to make it accessible. A second, largely revised, edition of the Critique was published in 1787.

Readers and reviewers generally failed to appreciate the originality of Kant's ideas. Readers interpreted Kant as saying something more familiar to them than what he was actually saying. The idea that rationalist metaphysics, which was the main occupation of philosophers in Germany at the time, could be dismissed entirely was too revolutionary a concept to catch on easily.

One of Kant's main troubles, it seems, was that he was taken for an idealist. Idealism is the doctrine that reality is dependent upon the mind. A common idealistic argument suggests that everything I know about the world I learn through the senses, and so the things I "know" are not external objects and phenomena, but just the report of my senses. My concept of the world, an idealist would argue, is based entirely on sensory images that exist only in my mind, and has at best a dubious connection with the things in themselves that exist in the world.

A famous proponent of this position is George Berkeley, an Irish bishop who argues that esse est percipi"being is being perceived." He asserts that chairs and tables and the like have no independent existence, that they only exist in the mind of someone who is perceiving them. He evades the odd claim that these things cease to exist when no one is perceiving them by positing the existence of God as a being who is perpetually perceiving everything.

Kant's philosophy is very firm in asserting that we can know only about appearances, and that we can know nothing about things in themselves. This assertion is enough to make Kant an idealist of sorts, but he wants to qualify this title of "idealism." He is not, like Berkeley, saying that only appearances exist: though we can know nothing about things in themselves, they are still a crucial part of his philosophy.

Kant calls his philosophy "transcendental" or "critical" idealism. The "transcendent" world of things in themselves is contrasted with the "immanent" world of appearances. Because he believes that things in themselves exist, his idealism believes in the existence of a "transcendent" world that is behind the world of appearances.

His idealism is "critical" because it is directed toward what we can know, not toward what exists. He is not saying that only appearances exist, but that appearances are all we can know. Kant's critical philosophy questions how we can come to know what we know, so he is an idealist only in saying that we cannot know things in themselves.

The concept of the thing in itself is one of the most controversial aspects of Kant's philosophy. In Germany, Kant's successorsmost notably Hegelcriticized this concept, and advanced a pure form of idealism that did away with things in themselves entirely. It is unclear what sort of relation things in themselves are supposed to bear to appearances if categories such as space, time, and causality do not apply to them. For instance, suppose we see Frank hit John, and then John hit Frank back, we must assume that these appearances are somehow related to things in themselves. But how can there be actions and reactions in things in themselves without the concept of time?

Because he says that they are unknowable, Kant cannot say anything about the nature of things in themselves, but his silence in this regard leaves us with a number of mystifying puzzles. How can appearances in space and time be related to things in themselves outside of space and time? Those who are dissatisfied with Kant generally identify their dissatisfaction with the unanswered questions raised by Kant's doctrine of things in themselves. Idealists generally deny that things in themselves exist, and realists generally assert that categories like space and time have more than just a subjective existence.

Kant's Tables of Categories

Logical Table of Judgments

1: As to Quantity Universal [all x are y] Particular [some x are y] Singular [the x is y] 2: As to Quality Affirmative [the x is a y] Negative [the x is not a y] Infinite [the x is a non-y] 3: As to Relation Categorical [x is y] Hypothetical [if x then y] Disjunctive [either x or y] 4: As to Modality Problematic [x is possibly y] Assertoric [x is y] Apodeictic [x is necessarily y]

Transcendental Table of the Concepts of the Understanding

1: As to Quantity Unity (Measure) Plurality (Quantity) Totality (Whole) 2: As to Quality Reality Negation LimitatioN 3: As to Relation Substance Cause Community 4: As to Modality Possibility Existence Necessity/VERSE

Pure Physiological Table of the Universal Principles of Natural Science

1: Axioms of Intuition 2: Anticipations of Perception 3: Analogies of Experience 4: Postulates of Empirical Thought in General

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Analytical Overview

Kant's philosophy has been called a synthesis of rationalism and empiricism. From rationalism he takes the idea that we can have a priori knowledge of significant truths, but rejects the idea that we can have a priori metaphysical knowledge about the nature of things in themselves, God, or the soul. From empiricism he takes the idea that knowledge is essentially knowledge of experience, but rejects the idea that we cannot learn any necessary truths about experience, and in doing so he rejects Hume's skepticism.

He is able to create this synthesis largely thanks to a radical reconception of the nature of knowledge that comes from from experience. Though empiricists and rationalists may have disagreed about the value or certainty of knowledge from experience, they both generally thought of the mind as a neutral receptor: knowledge from experience was simply the report of the senses. Kant points out that our knowledge of experience extends far beyond what the senses can report. Our senses can report sensations, but they cannot give these sensations a structure in space and time, or organize them according to cause and effect. According to Kant, our faculties or sensibility and understanding are largely responsible for what we think of as "knowledge from experience."

By giving our mind this complex internal structure, Kant makes room for a great deal of a priori knowledge. Though the sensations that are the basis for all our experience come from things in themselves, any regularity or structure we find in these sensations comes from our mental concepts. Thus, while Kant does not slip into the idealist position of saying that reality is all a matter of perception, he does claim that the laws of nature are the laws of our mental faculties. For something to be an objective law, it must be synthetic and it must be a priori, and Kant identifies the possibility of synthetic a priori knowledge within the structure of our mental faculties.

If our sense of order and regularity is not something we find in experience, but something we impose upon experience, the study of this order and regularity is a study of our own faculties rather than a study of experience. Kant reconceives the purpose of metaphysics as being one of critique: we must seek to understand how knowledge is structured, and consequently how the various concepts of our mental faculties are organized. This is an important step for philosophy: after Kant published his work, there was been less interest in making extravagant claims about the nature of the universe, and greater emphasis on determining what we can know and on what grounds we can claim to know it.

This kind of scrutiny, which Kant advocates, has led to some serious attacks on his work. The German Idealistsnotably Hegelwere the first to call into question Kant's concept of the thing in itself. Kant insists that while all we can perceive are appearances, these appearances are caused by things in themselves that are outside the realm of experience. Because they are outside the realm of experience, they are also outside the realm of space and time and any of the regularities we perceive in nature. A number of questions can be raised about what kind of relation things in themselves can have with appearances if categories such as time, causation, and even existence do not apply to them. The response of the German Idealists was to abandon the concept of things in themselves and assert that only appearances exist.

Analytic philosophy also got its start by criticizing Kant. This movement criticized in particular his category of the synthetic a priori. Frege was the first to point out that geometry is not synthetic a priori. Pure geometrywhich consists only of deductive inferencesis analytic, and empirical geometrywhich deals with what space is like in the real worldis known a posteriori. This position was given a boost by Einstein's relativity, which shows that space is very different from what we had assumed and our understanding of it is certainly not a priori.

Frege also complains that Kant's definition of analytic and synthetic judgments rests on the subject-predicate form of grammar, which is not a necessary part of the logical structure of language. Efforts to define and classify analytic and synthetic judgments have been a major pre-occupation of analytic philosophy, especially in the first half of the twentieth century.

Though many of Kant's doctrines have fallen into question, his exhortation toward critical philosophy remains with us. Perhaps his most lasting contribution is the setting up of a new standard for rigor and circumspection in philosophical investigations.

Study Questions

Kant claims in the preface that Hume interrupted his "dogmatic slumber." What was Kant's "dogmatic slumber" and how did Hume's attack on causal reasoning prompt Kant's critical philosophy?

Answer for Study Question 1 >>

Kant's philosophical development took place in the German tradition of rationalist metaphysics. In the Prolegomena, as in other of his mature works, Kant refers to this form of metaphysics as "dogmatic" because there is very little effort made to question the ground on which these metaphysical claims are justified. Hume awoke Kant from this "dogmatic slumber" (which is what Kant calls his metaphysical period in the preface) by showing the importance and the difficulty of justifying knowledge claims. Hume argues that we have no rational justification for believing that every effect has a cause, but that we simply believe this out of habit. Kant observes that Hume's reasoning could be applied generally to a priori knowledge, thus casting doubt on the rational justification of all metaphysics. Hume's skepticism prompts Kant to seek a more solid basis on which to ground metaphysics.

Close

What is the difference between the a priori/a posteriori distinction and the analytic/synthetic distinction? How do both of these distinctions differ from the distinction between necessary and contingent truths?

Answer for Study Question 2 >>

The a priori/a posteriori distinction has to do with cognitions, or things that we know. It distinguishes between knowledge I can have prior to any experience and knowledge I gain from experience. The analytic/synthetic distinction has to do with judgments. It distinguishes between judgments that are trivial and judgments that bring together two different concepts. The necessary/contingent distinction has to do with whether a certain fact could have been otherwise. A priori truths are generally considered necessary, since they do not seem to hinge on the particularities of experience. However, saying a truth is a priori is a matter of discussing how we know it, and saying it is necessary is a matter of discussing its relation to other truths and to the world.

Close

What is a "thing in itself"? Why can't we perceive it directly? How can we perceive it? How can we even know that things in themselves exist if we cannot perceive them?

Answer for Study Question 3 >>

Kant argues that while experience is made up entirely of appearances, these appearances are in some way caused by things in themselves. We cannot perceive things in themselves directly; what we perceive must first be interpreted by our senses, and then by our faculties of sensibility and understanding. Our senses and faculties are what make it possible to connect with the world outside our mind, but they also determine the way this connection is made. Though we cannot perceive things in themselves directly, we know they must exist because there must be some cause behind the appearances we meet with in experience. The existence of things in themselves is crucial to Kant's philosophy, but he insists that we cannot know anything about them.

Close

What is the difference between judgments of perception and judgments of experience? What is the significance of this distinction?

What is Kant's criticism of Hume? Based on your knowledge of Hume, do you think this criticism is correct?

What is the purpose of our faculty of reason? What can it do? What can't it do?

What is the source of the "antinomies" discussed as cosmological ideas? How are these antinomies resolved?

What is the distinction between limits and bounds? What is the significance in saying that math and science have limits and metaphysics has bounds?

What is a "critique"? What is the solution to the general question of the Prolegomena? How is it a solution?

In what way is Kant's "transcendental" idealism dependent on things in themselves? What problems are there with the doctrine of things in themselves? How might Kant defend this doctrine?

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Terms

Metaphysics - The field of philosophy that investigates the constitution, nature, and structure of reality. Metaphysics goes beyond physics to examine the reality behind the phenomenal world. It asks questions that cannot be verified in experience: "Does God exist?" "Is the soul immortal?" "What are the ultimate constituents of matter?" "How are mind and matter connected?" and so on. In the Prolegomena, Kant argues that this kind of "dogmatic" metaphysics can never arrive at satisfying answers because our faculty of reason cannot teach us anything about things in themselves. He tries to replace dogmatic metaphysics with his own critical metaphysics that sets about examining the constitution, nature, and structure of knowledge.

Analytic - A statement whose predicate concept is contained in its subject concept. An example is "all bachelors are unmarried." The concept of being unmarried is part of the concept of "bachelor," so the predicate does not say anything new. Instead, it offers an analysis of a part of the concept of the subject.

Synthetic - A statement whose predicate concept is different from its subject concept. Such a statement joins two different concepts together, and in doing so, produces new and interesting judgments. The Prolegomena makes much of synthetic judgments that can be known a priori, since they constitute mathematics, pure natural science, and metaphysics.

a priori - Knowledge that can be gained prior to any experience. Mathematics is a form of a priori knowledge, because we can sort out mathematical truths in our head. Kant also refers to a priori cognitions as necessary, since nothing in experience can possibly contradict them. Synthetic a priori judgments are thus important, since they are necessary and interesting truths that we can know prior to any experience.

a posteriori - In contrast with a priori cognitions, a posteriori cognitions consist of knowledge that we gain from experience. These generally have to do with facts about objects in the world, like "all swans are white."

Intuition - A translation of the German word Anschauung, this word means more exactly a perspective or a point of view. According to Kant, our faculty of sensibility is structured by intuitions. There are two kinds of intuition: pure and empirical intuitions. Our pure intuitions are our concepts of space and time that we apply to everything we perceive. Once we have applied our pure intuitions of space and time to sensations they become empirical intuitions, that is, sensations that exist in space and time. Kant argues that our pure intuitions of space and time can be exercised independent of experience, and serve as the basis for mathematics and geometry.

Concept of the understanding - These concepts, listed in Kant's table of categories, give a law-like structure to experience. While the empirical intuitions of our faculty of sensibility give us only subjective knowledge of experience, the faculty of understanding makes our empirical intuitions objective by applying to them universal concepts such as cause and substance. On their own, these concepts in their pure form serve as the basis for the general laws of pure natural science, such as "every effect has a cause."

Sensibility - The faculty that gives structure to the report of our senses. Our senses perceive things in themselves, and our faculty of sensibility applies our pure intuitions of space and time to give form to these sensations. Sensations combined with pure intuition makes empirical intuitions. The faculty of sensibility ensures that whatever we perceive we perceive in space and time.

Understanding - The faculty that gives an objective, law-like structure to our experience. Our faculty of sensibility gives us empirical intuitions, and our faculty of understanding applies to these intuitions the pure con