Logical and Rhetorical Fallacies Flashcards
Affirmation of the consequent (+example)
If A then B, and B is true. Therefore A is true. in symbols: A → B,B |/= A
Example. If it’s raining, you get wet. You get wet. Therefore it is raining.
Denial of the antecedent (+example)
If A then B, and A is false. Therefore B is false. In symbols: A → B,¬A|/= ¬B
Example. If you study logic, you’ll be good at reasoning. You don’t study logic. Therefore you’ll be no good at reasoning.
Argument from fallacy
If a premises of an argument is false, the conclusion is false. The argument is invalid (i.e. contains a fallacy), so the conclusion is false.
If a premise of an argument is false, then the argument cannot be sound, but it can nonetheless be valid. It is true, however, that if an argument is valid and has a false premise, then you cannot justify an assertion of the conclusion on the basis of the argument. If an argument is invalid, this says
nothing about the truth or falsity of the conclusion.
Affirming a disjunct
A or B is true, and A is true: therefore, B is false. In
symbols: A∨B,A|/= ¬B
Begging the question
the conclusion of an argument is smuggled in amongst its premises. Such an argument will not convince you, unless you already believe the conclusion, but in that case obviously you don’t need the ar- gument anymore. Such an argument is also called petitio principii. We noticed previously, however, that such an argument is in fact logically valid. It would be of the form ‘A, B, C, D are true. Therefore B is true’.
Ad hominem
attacking the person rather than the claims made by a person, for instance by pointing to a person’s behaviour or character; rejecting a proposition on the ground that it has been said by a certain person, rather than on the grounds of what the proposition says.
Argument from authority
accepting a proposition because it has been said by someone, rather than because of what it says.
