From Descartes to Locke

Sense Perception And

The External World

Descartes’ Third Meditation

•  Descartes’ aim in the third Meditation is to “demonstrate” the existence of God, using only what (after Med.’s 1 and 2) he knows with certainty.

•  He begins with a review where he says some revealing (and important) things about how he understands sense experience.

Med 3: Paragraph 1

“I will now shut my eyes, block my ears, cut off all my senses. I will regard all my mental images of bodily things as empty, false and worthless .... I will ... examine myself more deeply, and try ... to know myself more intimately. I am a thing that thinks, i.e that doubts, affirms, denies, ... [etc]. This thing also ... has sensory perceptions; ... even if the objects of my sensory experience ... don’t exist outside me, still sensory perception ..., considered simply as mental events, certainly do occur in me.”

What this means

•  I cannot trust the veracity of sense experience. •  But I know that I am “a thing that thinks.” – This means a thing with conscious mental states.

•  I know I have sensations “in my mind,” even if I don’t know whether the objects of these sensations do not exist “outside me.”

Med. 3, Paragraph 3

I previously accepted as perfectly certain and evident many things ...—the earth, sky, stars, and everything else that I took in through the senses—but in those cases what I perceived clearly were merely the ideas or thoughts of those things that came into my mind .... But I used also to believe that my ideas came from things outside that resembled them in all respects. .... [This] was false; or anyway if it was true it was not thanks to the strength of my perceptions.

Med. 3, Paragraph 6

“When ideas are considered solely in themselves and not taken to be connected to anything else, they can’t be false; for whether it is a goat that I am imagining or a chimera, either way it is true that I do imagine it. .... All that is left—the only kind of thought where I must watch out for mistakes—are judgments. And the mistake they most commonly involve is to judge that my ideas resemble things outside me.’

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What I really knew vs. what I thought I knew

•  I know that my ideas exist-- – whether of “the earth,” “goats,” or mere

“chimera” (i.e., non-existent beings)-- •  …because I directly perceive these “mental

contents.” •  But I merely “judge” (infer) that my ideas

“come from” things outside me which they “resemble.” – This is what makes “mistakes” possible.

What do I know? •  I know that I exist. –  I know that I am a “thinking thing,” a “mind.”

•  i.e., the subject of conscious experiences. –  Med. 2 and 6 argue that this “mind” is non-material.

•  I know I have ideas or sensations “in” my mind. –  These “mental contents” are what I “directly” or

“immediately” perceive.

•  I “judge” (i.e., infer) that these mental contents are caused by things that exist outside my mind, and that my ideas “resemble them.” –  This is what Med. 4-6 attempt to prove.

Descartes’ (Locke’s too) Theory of Perception: The mind perceives ideas which are caused by and

represent real objects.

Mind’s Eye Idea Object

Idea Object

Mind

Indirect Perception: a.k.a.

Representational Realism •  What I “directly” or “immediately” perceive

are always merely ideas “in” my mind. – These include sensations, or any “mental

contents.” •  I infer that these ideas were caused by, and

resemble, real objects that exist “outside” my mind.

Indirect Perception

•  If I am hallucinating (or if I am a brain in a vat), then my ideas are not caused by (or do not resemble) objects that exist outside my mind.

•  If I am not hallucinating, then these ideas do resemble “external” objects that caused them.

Indirect Perception

•  In both cases, what I immediately or directly perceive is the same—i.e., something that exists “in” my mind.

•  The difference between the two cases is in whether these ideas in my mind correctly “correspond” to a world outside my mind.

•  So, what we perceive are ideas, which, if we are not hallucinating, are caused by “real” objects outside the mind.

T.V. in your Head

•  When you watch T.V., you don’t really “see” human beings, you see images on a screen that were caused by (and resemble) human beings.

•  Indirect perception says the same is true of all perception. We don’t really “see” external objects. We see mental images that (we believe) were caused by (and resemble) external objects.

What do I see?

Descartes, Locke, Berkeley

•  All three accept (without much argument) that what we directly or immediately know are only “ideas” or other “mental contents.”

•  Descartes argues (in Med. 3-6) that there is a world outside our mind.

•  Locke accepts (without argument) that there is such a world, but claims that our sensations do not always resemble it.

•  Berkeley argues that there is no world outside mind (yours, mine, and God’s).

Terminology

•  Empiricism: –  All knowledge ultimately

rests upon sense experience.

–  Our justification for claiming we know something must always end up with something we perceive with our senses. •  “Seeing is believing.”

•  Rationalism: –  Not all knowledge

ultimately rests upon sense experience.

–  At least some (maybe all!) knowledge can be justified without appealing to sense perception. •  E.g., 2+2=4.

“Whose on Third?”

•  Descartes is a rationalist. –  He believes that there are some things we can know—some

beliefs that we can justify—without appealing to sense experience.

•  Locke and Berkeley are empiricists. –  They think all knowledge arises from sense experience. –  But they accept Descartes’ claim that what we directly

know, via the senes, are merely “ideas” or “sensations” that exist in the mind.

John Locke

An Essay Concerning Human Understanding

Locke’s Theory of Perception

•  “Whatsoever the mind perceives in itself, or is the immediate object of perception, thought, or understanding, that I call idea; –  So, what we immediately perceive (in any mental state) are

always “ideas.”

•  and the power to produce any idea in our mind, I call quality of the subject [object] wherein that power is.”

•  Objects (in the world) have the power to cause the ideas in our minds.

Locke’s Causal Theory of Perception:

Trees and Forests

•  Q: If a tree falls in the forest with no one there to hear it, does it make any sound? – A: Yes and no.

•  Yes—It “disturbs” airwaves. •  No—It doesn’t produce any sensations.

– What is a “sound?” •  Sometimes we use the word to talk about kinds of air

waves. •  Sometimes we use the word to talk about kinds of

sensations.

Qualities (or Properties):

•  Locke calls these “powers” –  by this he means whatever it is in the object

(whatever “power” it has) that causes us to have ideas of it. •  His causal explanation: objects emit tiny particles or “corpuscles” that enter our eyes and cause us to have certain ideas. •  Our explanation: objects reflect wavelengths of light

(which can also be considered tiny particles) that enter our eyes and cause us to have certain ideas.

Question: Do the ideas (in our minds that we directly perceive) resemble the qualities in the (external) objects that caused those ideas in our minds?

E.g., does a sensation of a green rectangle resemble its external cause?

Locke: Only partly. Our ideas of primary qualities

resemble their causes, but our ideas of secondary qualities do not.

“Consider the red and white colours in porphyry….”

•  “Hinder light from shining on it, and its colour vanishes –  It produces no idea in us

•  Upon the return of light it produces these same appearances in us

•  Can anyone think any real alterations are made in the porphyry by the presence or absence of light – when, … it has no colour in the dark?”

Other examples of the same principle:

•  The sound of an approaching or receding siren. –  The sound the siren produces doesn’t change, but the pitch

that we hear does. •  Put a hot hand in luke-warm water, and it will feel

cool. Put a cold hand in the same water, and it will feel warm. –  But the water (and all its qualities) haven’t varied. –  So, neither the (sensations of) coolness nor warmth (that you

feel) can resemble their causes “in” the water (because the sensations are different, but have the same cause).

Qualities: •  Primary Qualities:

–  Solidity, extension, figure, mobility, bulk, weight, texture

–  “are utterly inseparable from body…”

•  The ideas that primary qualities produce in us resemble those qualities in the objects that caused us to have those ideas.

•  Secondary Qualities: (“Sensible qualities”)— –  color, taste, smell, sound,

[felt] temperature –  are caused in us by collections

of primary qualities

•  The ideas that secondary qualities produce in us DO NOT resemble those qualities in the objects that caused us to have those ideas.

The crucial difference:

•  Objects have qualities, which are “powers” (capabilities) to produce ideas in us.

•  Sometimes the idea that an object produces in us resembles

the quality in the object that caused us to have that idea: THESE WE CALL PRIMARY QUALITIES.

•  Sometimes the idea that an object produces in us does not

resemble the quality in the object that caused us to have that idea: THESE WE CALL SECONDARY QUALITIES.

Are secondary qualities (really) in the mind?

•  NO! –  “Whatsoever the mind perceives in itself, … that I call

idea; and the power to produce any idea in our mind, I call quality of the [object] wherein that power is.” •  Ideas exist in minds. Qualities exist in objects.

•  Locke’s point is that when we perceive secondary qualities, that “power” in the object that causes us to have a sensation (i.e., a “combination” of primary qualities in the object) does not resemble the sensation that it causes us to have