Lecture 3 s 10 Fallacies 1

Lecture 3 s 10 Fallacies 1 - FALLACIES A logical fallacy is...

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FALLACIES A logical fallacy is an argument that contains a mistake in reasoning . Fallere (Latin) - to deceive Formal Fallacy : applicable only to deductive arguments There is an error in the structure of the argument: It contains an invalid rule of inference independent of content. Informal fallacy : Applicable to both de- and inductive requires knowledge of what is true in the real world.
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CATEGORIES – INFORMAL FALLACIES The informal fallacies we look at can be divided into five groups: Fallacies of relevance Premises irrelevant to conclusion Fallacies of weak induction Premises provide insufficient reason Fallacies of presumption Premises presume what they purport to prove Fallacies of ambiguity Either the premises or the conclusion is ambiguous (has more than one interpretation) Fallacies of grammatical analogy Argument grammatically analogous to valid one
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SOME VALID RULES OF INFERENCE A is true or A is false. (LAW OF THE EXCLUDED MIDDLE) A cannot be simultaneously true and false. A. Therefore, A or B. A. B. Therefore, A and B. A and B. Therefore, A. Not A. Therefore, not both A and B. A or B. Not A. Therefore, B. (DENYING DISJUNCT) Not (A and B). Therefore (not A) or (not B). (DE MORGAN) Not (A or B). Therefore (not A) and (not B). (DE MORGAN) If A then B. A. Therefore, B. (AFFIRMING THE PRECEDENT) If A then B. Not B. Therefore, not A. (DENYING CONSEQ.)
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SOME IN VALID RULES OF INFERENCE *A or B. Therefore, A. *A or B. A. Therefore, not B. (AFFIRMING THE DISJUNCT) *Not both A and B are true. Not A. Therefore B. (DENYING THE CONJUNCT) *If A then B. B. Therefore, A. (AFFIRMING THE CONSEQUENT) *If A then B. Not A. Therefore, not B. (DENYING THE ANTECEDENT) *If A then B. C. Therefore, B. *If A then B. not C. Therefore, not A. *If A then B. A. Therefore, C. *If A then B. not B. Therefore, not C.
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FALLACIES OF RELEVANCE Premises are logically irrelevant to the conclusion. Mary says X is true. Mary does Y. Anybody who does Y is a bad person. Therefore, X is false. The argument form of the argument is: if A then B. A. Therefore C. To conclude that X is false, we need an additional premise –”Everything a bad person says is false” (here, if B then C). The conclusion here (that Mary is bad) and ignores the difference.
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FALLACY OF WEAK INDUCTION The connection between the premises and the conclusion is not strong enough . All Ys are Zs. Mary says X is a Y. Therefore, X is a Z. The argument form is: If A then B. C. Therefore B. To conclude that X is a Z, we would need an additional Premise: If Mary says X is a Y, X is in fact a Y. The fallacy consists in treating one of the stated premises (Mary says X is a Y) as if it were a different premise (X is a Y).
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1. Appeal to force Arguer threatens opponent 2. Appeal to pity Arguer elicits pity from opponent 3. Appeal to the people Arguer arouses mob mentality or Appeals to opponent's desire for respect, love .
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This note was uploaded on 09/08/2010 for the course LING 21 at San Jose State University .

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Lecture 3 s 10 Fallacies 1 - FALLACIES A logical fallacy is...

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