1

Philosophy and ArgumentsPhilosophy and Arguments

2

OutlineOutline

1 – Arguments: valid vs sound

2. Conditionals

3. Common Forms of Bad Arguments

3

OutlineOutline

1 – Arguments: valid vs sound

1 – Arguments: valid vs sound

2. Conditionals

3. Common Forms of Bad Arguments

4

ArgumentsArguments

Argument: a sequence of sentences where one (the conclusion) is meant to follow from or be

supported by the others (the premises).

P1 All men are mortal.

P2 Socrates is a man.

C. Socrates is mortal.

Premise 1

Premise 2

Conclusion

Arguments can be:

1. Valid / Invalid

2. Sound / Unsound

5

Valid ArgumentsValid Arguments

Valid Arg.:

- The conclusion logically follows from the premises

- An argument where it is impossible for the premises to be true and the conclusion false.

P1 Jackie’s baby is a girl or a frog.

P2 Jackie’s baby is not a frog.

CC Jackie’s baby is a girl.

P1 All men are mortal.

P2 Socrates is a man.

CC Socrates is mortal.

VALIDITY = LOGICAL FORMVALIDITY = LOGICAL FORM

6

Valid ArgumentsValid Arguments

Attention!

- Just because an argument is valid, it does not mean that its conclusion is true

- Just because an argument is invalid, it does not mean that its conclusion is false.

P1 Jackie’s baby is a girl or a frog.

P2 Jackie’s baby is not a frog.

CC Jackie’s baby is a girl.

P1 No reptiles has big ears

P2 Bugs Bunny is not a reptile .

CC Bugs Bunny has big ears.

But if the premises are true and the argument is valid, then the conclusion is always true

But if the premises are true and the argument is valid, then the conclusion is always true

7

Sound ArgumentsSound Arguments

Sound Argument = VALID + TRUE PREMISSESSound Argument = VALID + TRUE PREMISSES

Sound arguments always have true conclusions.

8

Valid vs SoundValid vs Sound

Valid Arg which is not sound:

P1 Jackie’s baby is a cabbage or a frog.

P2 Jackie’s baby is not a frog.

C. Jackie’s baby is a cabbage.

Sound Arg:

P1 Jackie’s baby is a girl or a boy.

P2 Jackie’s baby is not a boy.

C. Jackie’s baby is a girl.

9

Conclusion: Good and Bad Conclusion: Good and Bad ArgumentsArguments

Good arguments are SOUND, ie are valid and possess true premises – They support the conclusion

Bad arguments are either:

- Invalid : the logical form is incorrect, or

- Unsound: the premises are false

These are the two ways in which you can criticize an argument from a philosopher !

To Remember: distinction valid / sound arguments

To Remember: distinction valid / sound arguments

10

OutlineOutline

1 – Arguments: valid vs sound

2. Conditionals 2. Conditionals

3. Common Forms of Bad Arguments

11

ConditionalsConditionals

Conditional : IF P THEN Q / P Q

Necessary and Sufficient conditions:

IF P THEN Q

P Sufficient Q Necessary

If you are pregnant then you put on weight

12

ConditionalsConditionals

Attention: IF P THEN Q is NOT equivalent to IF

Q then P

Attention: IF P THEN Q is NOT equivalent to IF

Q then P

If you are pregnant, you put on weight

Is NOT equivalent to:

If you put on weight then you are pregnant

13

ContrapositionsContrapositions

Contraposition: IF P THEN Q is equivalent to IF not Q

then not P

Contraposition: IF P THEN Q is equivalent to IF not Q

then not P

If you are pregnant, you put on weight

IS equivalent to:

If you DON’T put on weight then you are NOT pregnant

14

Conclusion on ConditionalsConclusion on Conditionals

To Remember:1. What a conditional is2. The distinction between necessary and

sufficient conditions3. The notion of contraposition

To Remember:1. What a conditional is2. The distinction between necessary and

sufficient conditions3. The notion of contraposition

15

OutlineOutline

1 – Arguments: valid vs sound

2. Conditionals

3. Common Forms of Bad Arguments3. Common Forms of Bad Arguments

16

Using a conditional the wrong wayUsing a conditional the wrong wayTwo ways to use a conditional the wrong way:

Affirming the antecedent / Denying the consequence

Two ways to use a conditional the wrong way: Affirming the antecedent / Denying the

consequence

Affirming the antecedent

PIf you are pregnant,

Hence Qthen you gain weight

Affirming the consequent

QIf you gain weight,

Hence Pthen you are pregnant

Denying the antecedent

Non QIf you are not pregnant,

Non Pthen you don’t gain weight

Denying the consequent

Non PIf you don’t gain weight,

Non Qthen you are not pregnant

IF P THEN QIf you are pregnant, then you gain weight

IF P THEN QIf you are pregnant, then you gain weight

17

Circular ArgumentsCircular Arguments

Circular arguments assume what they want to prove. Circular arguments assume what they want to prove.

The police did not beat the suspect because beating suspects in not something that police ever do.

Darwin's account of evolution is just a theory. A theory is an unproven hypothesis. So, there is no compelling reason to believe Darwin's theory.

18

Consequential FallacyConsequential Fallacy

Consequential fallacy: confuse the consequences of holding a belief with evidence for that belief.

Consequential fallacy: confuse the consequences of holding a belief with evidence for that belief.

Darwin’s theory is false because if it were true, there would be no morality.

19

EquivocationEquivocation

Equivocation: to use an ambiguous term in different ways in an argument.Equivocation: to use an ambiguous term in different ways in an argument.

Mad men should not be allowed to make decisions about the lives of others.

My father is mad.

He should not make an important decisions about my life.

20

Appeal to Consensus Appeal to Consensus

Appeal to Consensus: one appeals to consensus to establish a claim. Appeal to Consensus: one appeals to consensus to establish a claim.

Most people believe that McDonals is the best restaurant in the world. McDonalds is the best restaurant in the world.

21

Slippery Slope Slippery Slope

Slippery slope: incorrectly reasons that the arbitrariness of marking a distinction along some continuum shows that no distinction is possible.

Slippery slope: incorrectly reasons that the arbitrariness of marking a distinction along some continuum shows that no distinction is possible.

There is no agreed upon number of hair that qualifies someone as bald. Therefore, there is no difference between being bald or hairy.

22

Misleading VividnessMisleading Vividness

Misleading vividness: particularly vivid information is weighted more than other information in coming to a conclusion.

Misleading vividness: particularly vivid information is weighted more than other information in coming to a conclusion.

“Is there a post-abortion syndrome?” New York Times, January 21, 2007

Note that the argument can be criticized without criticizing the people who make the claims, nor even disagreeing with them about the moral status of abortion.

23

Genetic FallacyGenetic Fallacy

Genetic fallacy: taking the source of a claim as evidence for or against the claim.Genetic fallacy: taking the source of a claim as evidence for or against the claim.

Tom Cruise said that there is no such thing as a chemical imbalance of the body. Tom Cruise is crazy. Therefore, there is such thing as a chemical imbalance of the body.

Note that the argument can be criticized even if the conclusion is true.

24

Straw ManStraw Man

Straw man: misrepesenting someone’s position, arguing against it, and supposing that the actual position is defeated.

Straw man: misrepesenting someone’s position, arguing against it, and supposing that the actual position is defeated.

25

Straw ManStraw Man

Nigel: I believe that some kinds of sexual lifestyles are morally wrong.

Basil: So, you’re saying it’s OK for rednecks to beat up gay people?

Nigel: No, I’m not saying that at all. All people in our society should be protected from having unwanted violence inflicted upon them. I’m just saying I think their chosen lifestyles are immoral.

26

Straw ManStraw Man

Basil: What makes you think it’s OK for you to force your morality on everyone else?

Nigel: I haven’t said anything about forcing my morality on anybody. All I did was give my opinion about a certain moral issue. I didn’t use any force or even the threat of force to coerce others to agree with me.

27

Straw ManStraw Man

Basil: But you are saying that you don’t think gay people should have the same rights as straight people, right?

Nigel: No. I think all people in a democratic society should have the same rights. That means that people should have the right to pursue lifestyles that others think are immoral. I haven’t said anything about depriving people of their rights or inflicting violence upon them. I’m only giving my opinion about the morality of their behavior.

Bebee (2003) ``Good and Bad Arguments.’’

28

FeelingsFeelings

Feelings: a person argues for a position by indicating that they feel a certain way.Feelings: a person argues for a position by indicating that they feel a certain way.

Problem: No one cares how you feel.

Solution: Use good arguments!

More important problem: There is no warrant the think that your feelings about something are correlated to the truth or falsity of a proposition.