Introduction to Philosophy Lecture 2 A deeper look at arguments

1. Introduction to PhilosophyLecture 2A deeper look at arguments By David Kelsey

2. Conclusions as premises • Conclusions as premises: • The Margaret example (example 1), • 1) Every student who made 90 percent or better on the midterms already has an A. • 2) Margaret made a 94 percent on the midterms. • Thus, 3) Margaret already has an A. • 1) Margaret already has an A. • 2) All students who have an A are excused from the final exam. • Thus, 3) Margaret is excused from the final exam.

3. Unstated Conclusions • Unstated Conclusions: • Conclusions that are entailed by the words of the text. • The Republicans example (example 2): • 1) The political party that best reflects mainstream opinion will win the most seats in congress. • 2) The republicans certainly best reflect mainstream opinion. • What is the unstated conclusion?

4. IndependentPremises • Independent premises: independently support the conclusion of an argument.

5. The speed limit example • An argument with independent premises: • Speed limit example (example 3): • 1) Raising the speed limit will wear out the highways faster. • 2) Raising the speed limit will result in more highway deaths. • Thus, 3) We should not raise the speed limit.

6. Dependent premises: support the conclusion of an argument only with the help of some other premise. What if a dependent premise is false? Gas to waste (example 4): 1) Raising the speed limit will waste gas. 2) We don’t have any gas to waste. Thus, 3) We shouldn’t raise the speed limit. Dependent Premises

7. Good arguments • A good argument: • The premises both support its conclusion and are reasonable • Reasonable? • Requisite Support?

8. Valid arguments • Valid arguments • Necessary connection between premises and conclusion • The premises entail the conclusion

9. Valid arguments: false premises & false conclusions • We can have a valid argument with a false premise and a false conclusion. • George Bush example (example #5): • 1) George Bush is a robot. • 2) If George Bush is a robot then he isn’t the president. • Thus, 3) George Bush isn’t president. • And we can have a valid argument with false premises and a true conclusion: • The moon is made of green cheese example (example #6): • 1) The moon is made of green cheese. • 2) All things made of green cheese orbit the Earth. • Thus, 3) The moon orbits the Earth.

10. A validity metaphor • Form: • Valid arguments have a certain form to them • The link between premises and conclusion • The puzzle metaphor:

11. 2 more Validity tricks • 2 more tricks: • Addition metaphor: • Imagine an argument as a mathematical problem of addition. • Covering up the conclusion: • Cover up the conclusion and ask: what do the premises entail?

12. Sound Arguments • A sound argument: • The Giants example (example #7): • 1) The Giants won the superbowl. • 2) If a team loses a playoff game, then they don’t win the superbowl. • Thus, 3) The Giants didn’t lose a playoff game. • Dreaming example (example #8): • 1) Either I am in class right now or I am dreaming it. • 2) I am in class right now. • Thus, 3) I am not dreaming it.

13. Strong Arguments • A strong argument • April showers example (example #9): • 1) Every April we get some rain. • Thus, 2) We will get some rain this April.

14. Strong arguments: false premises & false conclusions • A strong argument with a false premise and a false conclusion: • Gravity example (example #10): • 1) Every time I have ever dropped anything from my hand it floats up to the sky. • Thus, 2) if I drop this pencil it will float up to the sky. • A strong argument with true premises and a false conclusion. • Swans example (#11): • 1) Every swan I have ever seen is white. • Thus, 2) all swans are white. • (All swans are not in fact white as there are black swans in Australia!)

15. Cogent arguments • A cogent argument: • 1. • 2. • The sun will rise & set tomorrow (example #12): • 1) Every day of my life the sun has risen in the morning and set in the evening. • Thus, 2) Tomorrow the sun will rise in the morning and set in the evening.

16. Deductive arguments: valid or intended to be valid. Necessary conclusions Which of our 12 examples are deductive? Inductive arguments: not valid. Strong? Likely conclusions Which of our 12 examples are inductive? Deduction and Induction

17. Unstated Premises again • Unstated premises again: • Identifying the unstated • Ask: What must be assumed to make it either valid or strong? • Add a general claim that connects the premises and conclusion • Cheating example (#12): • 1) This is the second time you caught her cheating. • 2) she shouldn’t pass the class. • This argument becomes valid when we add this premise: • 1.5)

18. Determining the Unstated premise • Now Ask: Is the assumption reasonable? • Yes or No? • Determining what must be assumed to make an argument strong

19. Unstated premises &Strong arguments • The bloodhound example (#13): • 1) Moore’s dog is a bloodhound. • Thus, 2) Moore’s dog has a keen sense of smell. • What do we need to add to make this argument valid? • What do we need to add to make this argument strong?