Syllogisms

Question 1

Categorical syllogism

Answer 1

Describe relation between two categories using all, no, or same

Syllogisms are valid if conclusion follows logically from its two premises
BUT, even if valid, syllogism may not be true

Question 2

Example

Answer 2

Premise 1: All birds are animals
Premise 2: All animals eat food
Conclusion: All birds eat food

Question 3

Example of valid but untrue syllogism

Answer 3

P1: All birds are animals
P2: All animals have 4 legs
C: All birds have 4 legs

Question 4

Example of invalid but reasonable-looking syllogism

Answer 4

P1: All students are tired
P2: Some tired people are irritable
C: Some of the students are irritable

Invalid because some of the tired people are not irritable, they may be the students

Question 5

Evans et al

Answer 5

Believable conclusions often judged more valid, even when syllogism is invalid

Question 6

Mental Model Approach

Answer 6

Not always easy to determine validity
Johnson-Laird: suggested most people form a mental model

Applied to the following syllogism:
P1: None of the artists are beekeepers
P2: All of the beekeepers are chemists
C: Some of the chemists are not artists

Question 7

Conditional syllogisms

Answer 7

1st premise is conditional (if p, then q)
2nd premise asserts truth of p or q
Conclusion asserts truth of other statement (besides one in second premise)

Question 8

modus ponens

Answer 8

P1: If I study, I’ll get a good grade
P2: I studied
C: I’ll get a good grade

Question 9

modus tollens

Answer 9

P1: If I study, I’ll get a good grade
P2: I didn’t get a good grade
C: I didn’t study

Question 10

Invalid

Answer 10

P1: If I study, I’ll get a good grade
P2: I got a good grade
C: I studied

P1: If I study, I’ll get a good grade
P2: I didn’t study
C: I didn’t get a good grade

Maybe I got a good grade without studying

Question 11

Wason Four-Card Problem

Answer 11

people better at judging conditional syllogism validity when put in familiar, real-world terms