Logic: The Language of Philosophy. What is Logic? Logic is the study of argumentation o In Philosophy, there are no right or wrong opinions, but there.

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Presentation transcript:

Logic: The Language of Philosophy

What is Logic? Logic is the study of argumentation o In Philosophy, there are no right or wrong opinions, but there are arguments that are better than others.

What is an “Argument”? Argument : a set of statements consisting of at least two premises and at least one conclusion. o Premises: reasons supporting the conclusion o Conclusion: the statement you are trying to prove

Philosophers write arguments like this: 1.Socrates is a man.(premises) 2.All men are mortal Socrates is mortal(conclusion)

Arguments must be valid and sound An argument is valid when the premises guarantee the conclusion o This is not subjective An argument is sound if (1) it is valid; and (2) the premises are true

Argument Forms I. Modus Ponens ( “ Affirming Mode ” ) P→Q1. If the ground is wet, then it was raining. P2. The ground is wet Q3. Therefore, it was raining.

Modus Tollens Modus Tollens Latin for “ Denying Mode ” P→Q1. If the ground is wet, then it was raining. ~Q2. It was not raining ~P3. Therefore, the ground is not wet.

Common Errors P→Q ~P Q ~Q P Denying the Antecedent Affirming the Consequent

Denying the Antecedent P→Q 1. If Socrates is French, then Socrates is mortal ~P 2. Socrates is not French ~Q 3. Socrates is not mortal NOT VALID!

Affirming the Consequent P→Q 1. If Socrates is French, then Socrates is mortal. Q 2. Socrates is mortal P 3. Socrates is French NOT VALID!

Chain Reasoning P  Q1. Nellie is a dog. Q  R 2. A dog is a mammal P  R3. Nellie is a mammal.

Disjunctive Syllogism P  Q1. Either the maid did it or the butler did it. ~P2. The maid did not do it Q3. Therefore, the butler did it.

Hypothetical Syllogism P→Q1. If Jim comes to the party, then Chris will come too. Q→R2. If Chris comes to the party, then Pat will come too P→R3. Therefore, if Jim comes to the party, Pat will come too.