Formal Fallacies

Question 1

What is a fallacy?

Answer 1

An argument which is:

• in some respect isn’t a good one
• likely to look like a good one
• flawed in some crucial respect

Question 2

What is a formal fallacy?

Answer 2

• when the flaw affects validity
• they don’t instantiate a valid pattern
• flaw lies in structure

Question 3

What is an informal fallacy?

Answer 3

• the flaw doesn’t affect validity
• formally valid (perhaps sound)
• flawed in other respects

Question 4

What does ‘non sequitur’ mean?

Answer 4

It does not follow

Question 5

What is a non sequitur?

Answer 5

Reveals that the conclusion does not follow from the premises

Question 6

Give an example of a non sequitur

Answer 6

Mr Smith says there is a ‘non sequitur’ in paragraph 8: he second sentence does not follow from the first

Question 7

What is another term for a non sequitur?

Answer 7

Equivocation

Question 8

Give an example of denying the antecedent

Answer 8

(1) If p, then q
(2) It is not the case that p
(3) It is not the case that q

Question 9

What does the fallacy of denying the antecedent combine?

Answer 9

(a) a conditional premise
(b) a second premise denying the antecedent of that conditional and purporting to derive:
(c) a conclusion that denies the consequent of the conditional

Question 10

How does the fallacy of denying the antecedent arise?

Answer 10

Is it the mistake of thinking that the negation of a sufficient condition implies the negation of what is conditional

Question 11

How does illicit inversion arise?

Answer 11

The mistake that underlined the false idea that a conditional sentence entails its inverse

Another formal fallacy

Question 12

Give an example of affirming the consequent

Answer 12

(1) If p, then q
(2) Q
(3) P

Question 13

What does the fallacy of affirming the consequent combine?

Answer 13

(a) a conditional premise
(b) a second premise affirming the consequent of that conditional, and purporting to derive:
(c) a conclusion that affirms the antecedent of the conditional

Question 14

How does the fallacy of affirming the consequent arise?

Answer 14

It is the mistake of thinking of thinking that the assertion of a necessary condition implies the assertion of what is conditioned

Question 15

How does illicit conversion arise?

Answer 15

It is the same mistake that underlines the false idea that a conditional sentence entails its converse

Question 16

What is an exclusive disjunction?

Answer 16

• it means ‘p or q’ not both

e. g ‘wanted dead or alive’

Question 17

What is an inclusive disjunction?

Answer 17

• it means ‘p or q’ or both

Question 18

Who wrote on the topic of ‘is’ and ‘ought’?

Answer 18

David Hume ‘A Treatise of Human Nature’ (1739)

Question 19

Name the six formal fallacies

Answer 19

• equivocation (non sequitur)
• denying the antecedent
• illicit inversion
• affirming the consequent
• illicit conversion
• affirming a disjunct

Question 20

Can a valid argument have false premises and conclusion?

Answer 20

Yes

Question 21

Can an invalid argument (non sequitur) have true premises and conclusion?

Answer 21

Yes

Question 22

Consider this argument:

(1) If your name is Amy, then you have the same name as the First Minister for Scotland
(2) Your name is Amy
(3) You have the same name as the First Minister for Scotland

Is this argument valid or sound?

Answer 22

• example of modus ponens
• if your name is Joan: both premises and conclusion are false
• if your name is Nicola: both premises are false but the conclusion is true
• if your name is Amy: one premise is false, one premise is true and the conclusion is true

In ALL cases the argument is valid

Question 23

Consider this argument:

(1) If your name is Theresa, then you have the same name as the British PM
(2) It is not the case that your name is Theresa
(3) It is not the case that you have the same name as the British PM

Is this argument valid or sound?

Answer 23

• example of (formal fallacy) denying the antecedent

- if your name is Arthur: both premises and conclusion is true

Question 24

Does the validity of an argument depend on the truth of its premises?

Answer 24

• no
• the only combination of truth and falsity that a valid argument can’t have is:
• premises all true and conclusion false