WEEK 6A. CHAPTER 9:ARGUMENTS IN ORDINARY LANGUAGE

READ Copi/Cohen, pp. 322-334



9.7. DISJUNCTIVE AND HYPOTHETICAL SYLLOGISMS

1. Remember that a syllogism is an argument with two premises and one conclusion. We have been studying one kind of syllogism thus far--the categorical syllogism, made up entirely of categorical statements. We are not going to look at syllogisms with different kinds of statements in them: disjunctive statements and hypothetical (or conditional) statements.



2. Disjunctive syllogisms have one premise that is a disjunctive statement and another premise that either affirms (restates) one side of that disjunctive statement (one side is a disjunct) or negates one side. Some of these syllogisms are valid and some are not. See the summary on page 325.



3. Hypothetical syllogisms contain at least one hypothetical statement. When both premises are hypothetical statements, it is a pure hypothetical syllogism. When one of the premises is a hypothetical statement and the other either affirms or negates one or the other of the parts of that hypothetical statement (the left side of one in the form of "If..., then...." is called the antecedent and the right side the consequent), then it is a mixed hypothetical syllogism. Again some of these are valid and some are not.



DO EXERCISES: pp. 326-30, ##1-20



9.8. THE DILEMMA

4. The dilemma is not technically a syllogism, since it has three premises. But it is a form of deductive argument. Its form is as follows:

If STATEMENT#1, then STATEMENT#2.

If STATEMENT#3, then STATEMENT#4

Either STATEMENT#1 or STATEMENT#3

Therefore, either STATEMENT#2 or STATEMENT#4

This form of argument is always valid. But a dilemma is often used to conclude to a set of alternatives neither of which is acceptable. If the conclusion is not acceptable, we cannot call the argument bad on the basis of validity; consequently we need to find another althervative to undermine the argument. The only alternative is to dispute the truth of one or more of the premises. A deductive argument is logically sound if its inference is valid and its premise(s) true; a dilemma of the above form can only be attacked for its premises' truth. We attack the disjunctive statement by "going between the horns" and attack one of the conditional statements (called "grasping the dilemma by the horns). See the examples in the book.



DO EXERCISES: pp. 334-337, ##1-10