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Syllogistic Errors

The fallacies in this section are all cases of invalid categorical syllogisms. A categorical syllogism is an argument consisting of exactly three categorical propositions (two premises and a conclusion) in which there appear a total of exactly three categorical terms, each of which is used exactly twice. For example, the classic: (1) All men are mortal. (2) Socrates is a man. Therefore, (3) Socrates is mortal.

Affirming the Consequent

Any argument of the following form is invalid: (1) If A then B (2) B Therefore, A

Drawing Affirmative Conclusion

The conclusion of a standard form categorical syllogism is affirmative, but at least one of the premises is negative.

Exclusive Premises

A standard form categorical syllogism has two negative premises (a negative premise is any premise of the form ‘No S are P’ or ‘Some S is not P’).

Existential Fallacy

A particular conclusion is drawn from universal premises.

Four Terms

The fallacy is committed when a standard form categorical syllogism contains four terms.

Illicit Major

The predicate term of the conclusion refers to all members of that category, but the same term in the premises refers only to some members of that category.

Undistributed Middle

The middle term in the premises of a standard form categorical syllogism never refers to all of the members of the category it describes.