Arguments 2018 Taylor Francis Argument Set of statementssentencespropositions

Arguments © 2018 Taylor & Francis

Argument • Set of statements/sentences/propositions, some of which are offered as evidence for others. • Conclusion: what the argument tries to prove or establish. • Premises: propositions used as evidence for the conclusion. • generally, well-accepted starting points • Evidence: considerations that make a belief more likely to be true. © 2018 Taylor & Francis

• Socrates is a man. • All men are mortal. • Therefore, Socrates is mortal. © 2018 Taylor & Francis

• Deductive argument: intended to render the conclusion certain. • truth of the premises would guarantee the truth of the conclusion • Inductive argument: intended to render the conclusion probable. • possible, even if extremely unlikely for premises to be true and conclusion false © 2018 Taylor & Francis

More About Deduction © 2018 Taylor & Francis

Good arguments • True conclusions • (important conclusions? ) • True premises • Strong conditional support • probability that conclusion is true if the premises are true © 2018 Taylor & Francis

• All members of species x have lungs. • y is a member of species x. • Therefore, y has lungs. • All members of species x have lungs. • y has lungs. • Therefore, y is a member of species x. © 2018 Taylor & Francis

• All members of species x have lungs. • y is a member of species x. • Therefore, y has lungs. valid • All members of species x have lungs. • y has lungs. invalid • Therefore, y is a member of species x. © 2018 Taylor & Francis

Validity • A valid argument is one in which: • it is logically impossible for the premises to be true and the conclusion false, together. • there are no background assumptions that would make the premises consistent with a false conclusion. • even an omnipotent being couldn’t make the premises true and conclusion false. © 2018 Taylor & Francis

Validity • Is about conditional support. • don’t worry about whether the premises are actually true • Holds in any context. • suspend all your background knowledge about the world, except what’s needed to understand the premises and conclusion © 2018 Taylor & Francis

• An invalid argument is one that isn’t valid • A sound argument: a valid argument with true premises • follows that it must have a true conclusion © 2018 Taylor & Francis

A valid argument can have: 1. A false premise 2. A false conclusion 3. All false premises and a true conclusion 4. The same sentence as one of its premises and as its conclusion • Anything except: all true premises and a false conclusion © 2018 Taylor & Francis

Logical terms • Validity of an argument often hinges on special, logical terms: • “all, ” “some, ” “and, ” “if…then…, ” “not, ” etc. © 2018 Taylor & Francis

• (P 1) All cetaceans are heterotrophs. • (P 2) Mauyak is a cetacean. • (C) Mauyak is a heterotroph. Heterotrophs Cetaceans Mauyak © 2018 Taylor & Francis Can’t place Mauyak in the Cetacean circle without putting her in the Heterotroph circle valid

• (P 1) All cetaceans are heterotrophs. • (P 2) Mauyak is a heterotroph. • (C) Mauyak is a cetacean. Heterotrophs Cetaceans Can place Mauyak in the Heterotroph circle without putting her in the Cetacean circle Mauyak © 2018 Taylor & Francis invalid

• All instances of the form • (P 1) All Cs are Hs. • (P 2) m is a C. • (C) m is an H. will have to be valid. © 2018 Taylor & Francis

Belief Bias © 2018 Taylor & Francis

• Anything with a motor needs oil. • Cars need oil. invalid • Therefore, cars have motors. Arguments whose conclusions you already believe are more likely to seem valid. • Anything with a motor needs oil. • Opprobines need oil. invalid • Therefore, opprobines have motors. © 2018 Taylor & Francis

Why Demand Validity? © 2018 Taylor & Francis

Enthymemes • Incompletely stated arguments • If we insist on validity, we ensure that all the reasons are made fully explicit. © 2018 Taylor & Francis

• (P 1) The fastest jet from Los Angeles to San Francisco takes 1 hour and 15 minutes. • (P 2) There is no faster way to travel from LA to San Francisco than by jet. • (C) There’s no way to travel from LA to San Francisco in less than 1 hour. presupposes which is currently true, but might not always be Insisting on validity forces us to make our presuppositions explicit. © 2018 Taylor & Francis