RI Critical Thinking Unit E (Evaluating Deductive Arguments) 2023

UNIT E Evaluating Deductive Arguments

VALIDITY AND SOUNDNESS ▪ Deductive arguments are evaluated on two grounds: truth of premises and validity ▪ Validity: a deductive argument is valid if the premises are true and it is impossible for the conclusion to be false. ▪ i.e. the conclusion has to be true if the premises are true ▪ Upshot: regardless of the content of the premises, deductive arguments can be evaluated for their validity ▪ A sound deductive argument is one which is both valid and has true premises

TEST FOR VALIDITY ▪ Assume that the premises are true: ▪ Is the conclusion conceivablyfalse? ▪ As opposed to plausible ▪ If yes, then the argument is invalid

WHICH IS VALID? ▪ Example 1 P1: The Prime Minister’s dog is infested with fleas. P2: Fleas are bacteria. C: The Prime Minister’ s dog is infested with bacteria. (P1-2) ▪ Example 2 P1: Colette owned a dog. P2: All French bulldogs are dogs. C: Colette owned a French bulldog. (P1-2)

WHICH IS VALID? ▪ Example 3 P1: All television networks are media companies. P2: NBC is a television network. C: NBC is a media company.(P1-2) ▪ Example 4 P1: All automakers are computer manufacturers. P2: United Airlines is an automaker. C: United Airlines is a computer manufacturer. (P1-2)

WHICH IS VALID? ▪ Example 5 P1: All banks are financial institutions. P2: Wells Fargo is a financial institution. C: Wells Fargo is a bank. (P1-2) ▪ Example 6 P1: Only banks are financial institutions. P2: Well Fargo is a financial institution. C: Wells Fargo is a bank. (P1-2)

WHICH IS VALID? ▪ Example 7 P1: Australia is surrounded by water P2: All islands are surrounded by water C: Australia is an island (P1-2) ▪ Example 8 P1: Ducks lay eggs. P2: Human beings are not ducks. C: Human beings don’t lay eggs. (P1-2)

WHAT ABOUT THIS? ▪ Example 9 P1: This argument is valid C: I am Goh Chok Tong (P1) Valid! But why? Do the test of validity – assume P1 is true But if P1 is true, then C must follow Hence, it is valid And thus, P1 is true Thus, the argument is sound!

TEST FOR SOUNDNESS ▪ 1) Is the deductive argument valid? ▪ 2) Are the premises (including implicit ones) all true? ▪ If any of the answers is a “no”, then the argument is unsound. ▪ i.e. an argument needs to be both valid and possesses true premises for it to be sound ▪ Upshot: possible for an argument to be valid and still unsound because of false premises.

SOME PERMUTATIONS Valid Invalid True premises True conclusion All wines are beverages. Chardonnayis a wine. Therefore, chardonnay is a beverage. [sound] All wines are beverages. Chardonnayis a beverage. Therefore,chardonnay is a wine. [unsound] True premises F alse conclusion None exist All wines are beverages. Ginger ale is a beverage. Therefore, ginger ale is a wine. [unsound] F alse premises True conclusion All wines are soft drinks. Ginger ale is a wine. Therefore, ginger ale is a soft drink. [unsound] All wines are whiskeys. Chardonnayis a whiskey. Therefore,chardonnay is a wine. [unsound] F alse