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Philosophy 103: Introduction to Logic
Syllogistic Fallacies

I. Venn diagrams and logical analogies are two of the three most common methods to test syllogisms. A third method is based on derived rules of validity.
A. We will look at some arguments that might initially seem to be valid, but are not so.
B. In each case, an informal explanation of its invalidity is described.
C. These six reasons are, in effect, rules of the syllogism. Here, again, we follow Copi's analysis. There are other sets of rules that equally apply to the analysis of the syllogism, and you might want to inquire into some of these other methods.
D. Corresponding to each rule of the syllogism is a fallacy (or fallacies) which is applied to all arguments that do not follow that rule.
E. Do not memorize the rules (or the rule numbers), but do learn the names of the fallacies. The names of the fallacies describe what it is that is mistaken about the argument.
F.  One way to think about the way a syllogism works is to conceptualize the general idea that two things related to the same thing might be related to each other.  The following mnemonic model might be helpful.

mechanism.gif (2077 bytes)

Since S is related to M, and P is related to M, then S ought to be related to P.

II. The Fallacies are explained individually on the following pages. 

Note:  Before you study these pages, know thoroughly the quantity, quality, and distribution Chart for Standard Form Categorical Propositions and Venn Diagrams for Syllogisms.

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