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.