Philosophy 148

1. Philosophy 148 Chapter 7

3. Vocab: • We say that two categorical claims are ‘corresponding claims’ if and only if they have the same subject and the same predicate. • Do corresponding claims need to have the same meaning or truth value? (no)

4. Translation into categorical form • ‘S’ will generally be short for ‘subject’ and ‘P’ will be short for ‘predicate’. • S and P will both have to turn into plural noun phrases so that each circle on our diagrams will represent a class of things. • Claims about individuals can be made into plural noun phrases by adding a phrase like “persons identical with” or “things identical with”, etc. • Adjectival or verb phrases can be made into noun phrases by addition of phrases like “things that…”. For example, “everything green goes ‘splat’” turns into “All green things are things that go splat.”

5. Translation into A claims • Every S is P All of these go • Any S is P straight into the • Anyone S is P form: All S are P • Each S is P

6. Only the only • “Only x are y” translates to “All y are x”. ‘Only’ indicates the predicate of an A claim. • “x are the only y” translates to “All y are x” because ‘the only’ indicates the subject of an A claim

7. Contradictories • It is easy to see from diagrams that E and I claims contradict one another, that is, they cannot have the same truth value at the same time. The same holds for the A and O claims.

8. Existential Commitment • Since existential commitment doesn’t make a difference to most syllogisms (arguments constructed out of A, E, I, and O claims) we don’t have to make a decision at all in most cases. Where a decision is required, assume that people DO make existential commitments. Otherwise, our Venn diagram system will be an incomplete system.

9. Square of Opposition

10. Immediate Inferences • Immediate inferences are not really arguments because they have only one premise. • The purpose of evaluating inferences is to show whether or not we can get from point A to point B so to speak in a logically acceptable manner. • Another way of putting it is that a valid immediate inference is where one thing follows from another. An invalid immediate inference is where one thing does not follow (Latin: non sequitur) from another.

11. More immediate inferences • Before going on to a few types of immediate inference, here’s some vocab: • Complementary class: The class that is outside the class to which you were referring (It is easiest to just use the prefix ‘non-’ to represent the complementary class.) • Quality: Refers to whether the categorical claim is affirmative or negative.

12. Conversion • To convert a claim, switch around its subject and predicate terms. • That switched-around claim is called the converse of the original claim. (bad philosophy joke: Converse shoes are shoes made for the wrong feet.) • Conversion is a valid inference for E and I claims but not for A or O claims. • Seeing everything you eat is not the same as eating everything you see.

13. Obversion • To find the obverse of any given claim: • 1. Reverse the quality of the proposition (if it is an A, it becomes an E, if it is an I, it becomes an O, etc…) • 2. Replace the predicate with its complementary term (stick a ‘non-’ on it). • Obversion is always a valid immediate inference.

14. Contraposition • To find the Contrapositive of a claim: • 1. Convert the claim • 2. Replace both terms with their complementary terms • Contraposition is valid for A and O claims, but not for E and I claims.

15. Syllogism • A syllogism is made up entirely of categorical claims • There are 2 and only 2 premises and one conclusion • Major premise: contains the predicate term • Minor premise: contains the subject term • There are 3 and only 3 terms in a syllogism. • Predicate term: predicate of the conclusion • Subject term: subject of the conclusion • Middle term: term that is not in the conclusion, but is in each premise

16. Validity for Syllogisms • Validity means the same thing it always has. IF the premises are true, then the conclusion must be. • To determine if a syllogism is valid, we make a Venn Diagram with three circles and then diagram the premises. If the information given by the conclusion is represented by diagramming the premises, then the premises guarantee the conclusion, and the argument is valid.

17. Venn Diagram Method: • Draw three circles in the appropriate way • Label each circle in the appropriate way • Diagram the premises • If any area is the only un-shaded area of its circle, put a * in it. (existential commitment) After this, put your pen/pencil down. • Check if the information given by the conclusion is or is not represented on the diagram. If it is, the argument is valid, if not, the argument is not valid.

18. Note about the * • When the * could go in one of two areas on a three-circle Venn Diagram, it must go on the line between them to indicate that the * is in one of the two areas, but that the premises do not specify which. • The information that this conveys is NOT that there is a * in both areas, nor neither, but rather a single * possibly in either area. • See p.193-194

19. Just for fun… Medieval students used to have to memorize this chart to determine the validity of syllogisms