1 nondeductive logic induction & informal fallacies


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  • *ReasoningTo give an argument is to give reasons for a conclusion. Its more than offering an opinion or making an assertion.Reasoning is about what people should believe based on EVIDENCE. Rational inquiry depends on logic. Good arguments should persuade but they may not. That a good (sound) deductive argument does not persuade is a rhetorical issue, or a case of stubbornness on the part of the listener. Bad or fallacious arguments are often highly persuasive.

  • *Inductive Reasoning-1While a valid deductive argument is risk free since its conclusion follows necessarily from its premises, an inductive argument is risky. Its conclusion can be denied without contradiction because the conclusion is not entailed by its premises but the premises provide strong evidence for it.Most horses are friendly. Mr. Ed is a horse. Therefore, Mr. Ed is friendly.A deductive argument has All instead of most.

  • *Inductive Reasoning-2Induction is the process of drawing generalizations from known facts or research to give strength and support to conclusions.Inductive reasoning is the process of finding truth by making observations; inferring general laws and truths from specific instances.Inductive generalizations are possible, but some are much more probable than others.Statistical evidencedata collected by polling and research studiesleads to statistical generalizations, which are inferences drawn from statistical evidence used to give strength to inductive arguments.

  • *Statistical ReportsCharacteristic of interestthe specific question the researcher seeks to answer concerning a given population.Target populationthe group about which a researcher wishes to generalize.Samplemembers of the target population studied by the researcher.The sample must be representative [not biased] of the target population and must be random.

  • *Inductive Reasoning: TypesEnumerative inductionone or more premises state that some things of a certain kind have a certain feature, and this is offered as evidence that all things of the same kind have that same feature. Inductive generalization. (universal)Every raven so far observed has been black.Ravens are blackMany roses have been observed to bloom in the summer.All roses blossom in the summer.Counterexample proves the conclusion false.In the past, most animal species have survived by adapting.Animal species will continue to survive by adapting?

  • *Inductive Reasoning: TypesStatistical syllogisman inductive argument whereby a certain feature is ascribed to some case or cases on the basis of their being a part of a larger class, some of which, perhaps many have the ascribed feature. Most surgeons carry malpractice insurance.Dr. Marreno is a surgeon.Dr. Marreno carries malpractice insurance.Most As are BscompareAll As are Bsh is an Ah is an Ah is a Bh is a BInductivedeductive

  • *Inductive Reasoning: TypesStatistical syllogisms share a common pattern which can be used for explaining the past and the future. Most famous battles involved careful strategy.Trafalgar was a famous battle.Trafalgar involved careful strategy.80% of police officers have terrorism training.David will be a police officer.David will have terrorism training.n% of As are Bsm is an Am is a Bn
  • *Inductive Reasoning: TypesIn a causal argument, one or more premises are offered to support that the hypothesis that a certain event is causally related to another event. A cause regularly precedes an effect, but a cause must do more. Whenever something occurs, it is often possible to narrow the range of hypotheses about its likely cause. Mill identified two such methods: the method of agreement and difference and the method of concomitant variation. Agreement and Difference: What different occurrences of a certain phenomenon have in common is probably its cause. And factors that are present only when some observed phenomenon occurs are probably its cause.Concomitant Variation: When variations of one sort are highly correlated with variations of another, one is likely to be the cause of the other, or they may both be caused by something else.

  • *Inductive Reasoning: TypesThe Method of Agreement and Difference: An example: Coach wants to find out why three of his best players perform poorly on Friday afternoons.1David, Jim, and Bob have been performing poorly on Friday afternoons.2Going to late parties on Thursday is the one and only thing that all three do when and only when they perform poorly.3Going to late parties on Thursday likely causes their poor game performance.X has occurred several times.Y is the one and only other thing that precedes all occurrences of X.Y causes X.Coach must compare both agreement and differenceemploy jointly.

  • *Inductive Reasoning: TypesAnalogya type of inductive argument whereby a certain conclusion about an individual or a class of individuals is drawn on the basis of some similarities that an individual or class has with other individuals or classes: Davids vehicle, a 2007 Hummer, is expensive to run.Janes vehicle , a 2007 Hummer, is expensive to run.Marks vehicle, a 2007 Hummer, is expensive to run.Jacks vehicle is a 2007 Hummer.Jacks vehicle is expensive to run.

  • *Inductive Reasoning: TypesFactors that effect the success of an analogy:the number of things and features held to be analogous.the degree of similarities and differences among those things.the relevance of ascribed features to the hypothesis.the boldness of the hypothesis with respect to the evidence.Extensive research on polar bears and hippos has shown that they have a great number of relevant features in common with other species listed as threatened species. These threatened species might disappear. So polar bears and hippos might disappear.The pattern of reasoning underlying this analogy is:Things a and b have n relevant features in common with x, y, and z.x, y, and z also have feature n+1a and b probably have feature n+1.

  • *Inductive Reasoning: Types1--Anyone wishing to be a space tourist will have to pay $100 million. For, according to my records, each space tourist has recently paid $100 million for traveling in space.2-- Smaller, more fuel-efficient vehicles are now quite popular among many college students. My roommate is about to buy a vehicle. I predict that hell buy a small, fuel-efficient car.3--People who have a high percentage of folic acid in their diets also have a lower incidence of Alzheimers disease. But these people generally have more healthy habits. Thus either a high percentage of dietary folic acid causes a lower incidence of Alzheimers disease or having healthy habits causes both.4--The earth is a planet with carbon based life. The required elementsCarbon, water, and energyappear to be, or to have been, present on Mars, which is also a planet. This suggests that Mars could have had carbon based life.

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    Inductive Reasoning: Characteristics

    Inductive reliabilityan inductive argument is reliable if and only if its form is such that, if its premises were true, it would be reasonable to accept its conclusion as true. (cogency)99% of As are Bsh is an Ahighly reliableh is a B39% of As are Bsh is an Aless reliable (a matter of degree)H is a BCompare to validity of deductive argumentsnot necessarily truth preserving.

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    Inductive Reasoning: Characteristics

    Inductive strengthwhen an inductive argument is strong, it has a reliable form and its premises are true. It is then reasonable to accept its conclusion as true.80% of students in Biology 100 are women.Robin is a student in Biology 100.Robin is a women. An inductive argument is strong if and only if its hypothesis is the one that has the greatest probability of being true on the basis of the evidence. Compare inductive strength and deductive soundnessnot necessarily true but probably true.

  • *Definition of Informal fallacyA fallacy is a mistake in reasoning. A fallacious argument is one that contains such an error. To identify errors in reasoning is a necessary condition for reasoning soundly. In formal logic arguments are valid or invalid. We have discussed, for example, certain formal fallacies that arguments may have that make them invalid.An informal fallacy is a type of argument which, although incorrect, may nevertheless be psychologically persuasive either because we are deceived by some ambiguity in the language or we are misled by some technique designed to cause us to accept a particular conclusion, even though the truth of that conclusion is not established by the premises of the argument.Some fallacious patterns of reasoning are so common that they are given names. (20)

  • *Types of fallaciesFallacies of failed induction (5)Fallacies of presumption (4)Fallacies of relevance (6)Fallacies of ambiguity (5)Twenty discussed.

  • *Fallacies of failed induction (5 types)An argument can be classified as a fallacy of failed induction if its premises are irrelevant to its conclusion and therefore incapable of establishing the truth of its conclusion.Hasty generalizationfalse (weak) analogyPost hoc ergo propter hocAppeal to ignorance (ad ignorantiam)appeal to authority (ad verecundiam)

  • *Fallacies of failed induction (1)Hasty generalization (conclusion)the fallacy of trying to draw a conclusion about all things of a certain kind on the basis of having observed only a few of them, or some unrepresentative sample of them. This fallacy is committed by any enumerative induction whose conclusion rests on a sample that either is too small or lacks comprehensiveness and randomness, or both.A Midwesterner visits California and observes that three people she meets all practice yoga All Californians practice yoga.

  • *Fallacies of failed induction (2)False (wea


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