1. Last week Change minds; influence people Premises Conclusion
Argument in ordinary life:
Change minds; influence people
Beliefs you already have
Premises
(= Assumptions)
Beliefs I want you to have
Conclusion

2. Philosophy is like ordinary life: Using arguments to change minds.

3. Philosophy is like ordinary life: Using arguments to change minds.
Philosophy is not like ordinary life:
Using arguments for theoretical activity.

4. Theoretical Activity
Asking just to know—without worrying about how answers can be used or applied.

5. Theoretical Activity
Asking just to know—without worrying about how answers can be used or applied.
Doesn’t mean theories have no application, just that we aren’t thinking about applications while theorizing.
It wasn’t as though Edison had the idea of an electric light bulb first, and then went looking for evidence of electrical forces to power his light bulbs.

6. Theoretical Activity
Asking just to know—without worrying about how answers can be used or applied.
Doesn’t mean theories have no application, just that we aren’t thinking about applications while theorizing.
It wasn’t as though Edison had the idea of an electric light bulb first, and then went looking for evidence of electrical forces to power his light bulbs.
Without theoretical activity, we
wouldn’t have electricity (for example).

7. When you’re asking just to know, there’s no need for rhetoric.

8. When you’re asking just to know, there’s no need for rhetoric.
Rhetoric is “tricky” persuasion:
trying to convince people of things whether or not they’re really true. (e.g., by appealing to emotion).

9. When you’re asking just to know, there’s no need for rhetoric.
Rhetoric is “tricky” persuasion:
trying to convince people of things whether or not they’re really true. (e.g., by appealing to emotion).
“Michael Jackson didn’t do anything wrong. He’s the greatest singer ever!”

10. Conclusion: Michael Jackson didn’t do anything wrong.
Premises: 1) Michael Jackson is the greatest singer ever.
Conclusion: Michael Jackson didn’t do anything wrong.

11. Conclusion: Michael Jackson didn’t do anything wrong.
Premises: 1) Michael Jackson is the greatest singer ever.
Assumption:
2) If Michael Jackson is a great singer, then he
wouldn’t do things that are wrong.
Conclusion: Michael Jackson didn’t do anything wrong.

12. Conclusion: Michael Jackson didn’t do anything wrong.
Premises: 1) Michael Jackson is the greatest singer ever.
Assumption:
2) If Michael Jackson is a great singer, then he
wouldn’t do things that are wrong. ?
Conclusion: Michael Jackson didn’t do anything wrong.

13. Conclusion: Michael Jackson didn’t do anything wrong.
Premises: 1) Michael Jackson is the greatest singer ever.
Assumption:
2) If Michael Jackson is a great singer, then he
wouldn’t do things that are wrong. ?
1a) Great singers make people happy.
2a) People who make people happy don’t do
things that are wrong.
Ca) If MJ is a great singer, then he wouldn’t do
things that are wrong.
Conclusion: Michael Jackson didn’t do anything wrong.

14. Conclusion: Michael Jackson didn’t do anything wrong.
Premises: 1) Michael Jackson is the greatest singer ever.
Assumption:
2) If Michael Jackson is a great singer, then he
wouldn’t do things that are wrong. ?
1a) Great singers make people happy. X
2a) People who make people happy don’t do
things that are wrong.
Ca) If MJ is a great singer, then he wouldn’t do
things that are wrong.
Conclusion: Michael Jackson didn’t do anything wrong.

15. Rhetorical arguments “pull the wool over the eyes” of the audience.
It wasn’t as though Edison had the idea of an electric light bulb first, and then went looking for evidence of electrical forces to power his light bulbs.

16. Rhetorical arguments “pull the wool over the eyes” of the audience.
Useful in practical activities...
-advertising
-politics
-law
It wasn’t as though Edison had the idea of an electric light bulb first, and then went looking for evidence of electrical forces to power his light bulbs.

17. Rhetorical arguments “pull the wool over the eyes” of the audience.
Useful in practical activities...
-advertising
-politics
-law
But not theoretical activities.
If you’re asking just to know, you don’t want to trick yourself. I

18. Three Types of Argument
Last week I asked:
“What connects the premises with the conclusion?”
Probability comes in degrees, of course.

19. ? Beliefs you already have Beliefs I want you to have
What does already believing the premises have to do with accepting the conclusion?

20. ? Logic. Beliefs you already have Beliefs I want you to have
What does already believing the premises have to do with accepting the conclusion?

21. ? Logic. Well, I lied—sort of... Beliefs you already have
Beliefs I want you to have
What does already believing the premises have to do with accepting the conclusion?
Well, I lied—sort of...

22. ? Logic. There are actually 3 ways for premises
Beliefs you already have
Logic.
Beliefs I want you to have
What does already believing the premises have to do with accepting the conclusion?
There are actually 3 ways for premises
to be “attached” to conclusions.

23. There are actually 3 ways for premises
Logic
Deductive
argument
Beliefs you already have
Inductive
Argument
Generalization
Beliefs I want you to have
Explanation
Abductive
Argument
What does already believing the premises have to do with accepting the conclusion?
There are actually 3 ways for premises
to be “attached” to conclusions.

24. Three Types of Argument
Deductive
If premises are true, conclusion must be true
Inductive
If premises are true, conclusion is probably true
Probability comes in degrees, of course.
Abductive (different)
The conclusion explains the premises.

25. Deductive Arguments

26. Deductive Arguments
The Ideal:
Logically valid
All premises true

27. Deductive Arguments The Ideal: Real Life: Logically valid
All premises true
Real Life:
Logically valid
All premises agreed upon
between arguer and audience

28. Deductive Arguments
Validity
Truth

29. Both of these are valid:
Deductive Arguments
Validity
Truth
Both of these are valid:
All fish swim
All sharks are fish
All sharks swim

30. Both of these are valid:
Deductive Arguments
Validity
Truth
Both of these are valid:
All fish swim
All sharks are fish
All sharks swim
All fish wear gold chains
All sharks are fish
All sharks wear gold chains

31. Both of these are valid:
Deductive Arguments
Validity
Truth
Both of these are valid:
All fish swim
All sharks are fish
All sharks swim
All fish wear gold chains
All sharks are fish
All sharks wear gold chains

32. Validity Truth Deductive Arguments
An argument can be invalid even when every statement in it is true.
All fish swim
All sharks are fish
All pimps wear gold chains

33. Validity Truth Deductive Arguments Valid or Invalid True or False
-Inferences
True or False
-Premises
-Assumptions
-Claims
-Beliefs
-Ideas
-Sentences

34. Deductive Arguments Quick Quiz:
Last week I said that if an argument makes assumptions, the assumptions are necessary parts of the argument. Why are they necessary?
A) Without them, the premises won’t be valid.
B) Without them, the conclusion won’t be true.
C) Without them, the argument won’t be true.
D) Without them, the argument won’t be valid.

35. Deductive Arguments Begging the Question (p. 19):
God exists because the bible says so
2) Consumer Reports is reliable because it rates itself as reliable.

36. Deductive Arguments “circular reasoning”?
Begging the Question (p. 19):
God exists because the bible says so
2) Consumer Reports is reliable because it rates itself as reliable.
Write out premises and conclusion.
What’s the technical meaning of
“circular reasoning”?

37. Inductive Arguments

38. If the premises are true, then the conclusion is probably true.
Inductive Arguments
If the premises are true, then the conclusion is probably true.

39. Deductive inferences follow logically
Inductive Arguments
Deductive inferences follow logically
If Tim is taller than Andy and Andy is taller than
Bill, it’s impossible for Bill to be taller than Andy.

40. Inductive Arguments Deductive inferences follow logically
If Tim is taller than Andy and Andy is taller than
Bill, it’s impossible for Bill to be taller than Andy.
Inductive inferences don’t

41. Inductive Arguments Deductive inferences follow logically
If Tim is taller than Andy and Andy is taller than
Bill, it’s impossible for Bill to be taller than Andy.
Inductive inferences don’t
If a woman isn’t wearing a wedding ring, it’s still
possible that she’s married.

42. Induction = Generalization
Inductive Arguments
Induction = Generalization
Essential to science:
If this salt dissolves in water, then all salt dissolves
in water.
Inferences about the world in general
are drawn from a particular sample.

43. “2 + 2 = 4” Inductive Arguments Mathematical claims are
verified by deduction...
“2 + 2 = 4”
Scientific claims are
verified by induction.
“Kangaroos don’t lay eggs.”

44. Inductive Arguments
Deductive inferences are either valid or not—validity doesn’t come in degrees.
Inductive inferences do come in degrees—some generalizations are stronger than others.

45. Inductive Arguments Inductive Strength 1) Sample Size
2) Bias (is sample representative?)

46. Abductive Arguments The conclusion explains the premises.
“Inference to the Best Explanation”

47. Abductive Arguments The conclusion explains the premises.
“Inference to the Best Explanation”
‘Inference’ is better than ‘argument.’
You can state an inference as an argument, but it’s less useful.

48. The conclusion explains the premises.
Abductive Arguments
The conclusion explains the premises.
Mendel’s inference:
Genes explain patterns of inheritance in pea plants.

49. The conclusion explains the premises.
Abductive Arguments
The conclusion explains the premises.
Mendel’s inference:
Genes explain patterns of inheritance in pea plants.
Not an inference about genes in general drawn from observations of particular genes.

50. Abductive Arguments The conclusion explains the premises.
Mendel’s inference:
Genes explain patterns of inheritance in pea plants.
Not an inference about genes in general drawn from observations of particular genes.
Mendel never observed a gene.

51. Abductive Arguments Likewise,
Copernicus didn’t observe the planets orbiting the sun.

52. Abductive Arguments Likewise,
Copernicus didn’t observe the planets orbiting the sun.
Newton didn’t observe gravity moving apples toward the ground.

53. Abductive Arguments Likewise,
Copernicus didn’t observe the planets orbiting the sun.
Newton didn’t observe gravity moving apples toward the ground.
Darwin didn’t observe natural selection.

54. Why “inference to the best explanation”?
Abductive Arguments
Why “inference to the best explanation”?
Abductive inferences involve hypotheses, and some hypotheses are better than others.

55. Why “inference to the best explanation”?
Abductive Arguments
Why “inference to the best explanation”?
Abductive inferences involve hypotheses, and some hypotheses are better than others.
GOOD
IF (hypothetically) the sun orbits the earth, THEN we’d observe what we actually observe.

56. Why “inference to the best explanation”?
Abductive Arguments
Why “inference to the best explanation”?
Abductive inferences involve hypotheses, and some hypotheses are better than others.
GOOD
BETTER
IF (hypothetically) the sun orbits the earth, THEN we’d observe what we actually observe.
IF (hypothetically) the earth orbits the sun, THEN we’d observe what we actually observe.

57. Abductive Arguments are Weird
Like inductive arguments, they come in degrees.

58. Abductive Arguments are Weird
Like inductive arguments, they come in degrees.
But unlike inductive arguments, their strength doesn’t depend on the strength (probability) of the premises.

59. Abductive Arguments are Weird
Like inductive arguments, they come in degrees.
But unlike inductive arguments, their strength doesn’t depend on the strength (probability) of the premises.

60. Abductive Arguments are Weird
An abductive argument just says the conclusion is one explanation of the premises.
It doesn’t say how good that explanation is, or whether a better one is available.

61. The Surprise Principle
Abductive Arguments
The Surprise Principle
To evaluate an abductive inference (a hypothesis), you can’t just look at whether any prediction it makes is true...
EXAMPLE:
Hypothesis: the patient is having a heart attack
Prediction: the patient has a heart

62. Abductive Arguments The Surprise Principle
You have to consider predictions that distinguish the hypothesis from other explanations.
EXAMPLE:
Alternative Hyp: the patient isn’t having a heart attack.
...we’d still predict that the patient has a heart.

63. The Surprise Principle
Abductive Arguments
The Surprise Principle
You have to look at surprising predictions—predictions you wouldn’t make without that hypothesis.
A “prophet” who predicts what everyone already expects to happen has no special powers.

64. The Surprise Principle
Abductive Arguments
The Surprise Principle
For a hypothesis to be supported:
No false predictions
Some true predictions are surprising

65. Abductive Arguments The “Only Game in Town” Fallacy
What if you only have one hypothesis, so you can’t make comparisons?

66. Abductive Arguments The “Only Game in Town” Fallacy
What if you only have one hypothesis, so you can’t make comparisons?
You can evaluate explanations by themselves, so you don’t have to accept a bad explanation just because it’s “the only game in town.”

67. Abductive and Inductive
Arguments
An Investment Swindle (p. 37)...
30 SLIDES IN 30 MIN

68. Abductive and Inductive
Arguments
An Investment Swindle (p. 37)...
Ten predictions in a row were correct.
These ten are a representative sample. (Assum.)
The investment firm has some way of knowing how well stocks will do.
Hyp 1
Ten predictions in a row were correct.
The investment firm got lucky ten times in a row.
Hyp 2