Rule

Violations are called … 

Discussion

1

There can only be three terms.

The four-term fallacy

Often caused by using different wording for one of the terms. However, the two wording have slightly different meanings.

2

The middle term must be universally distributed at least once.

Fallacy of the undistributed middle

Be sure to mark on the syllogism which terms are undistributed.

3 

Both premises can not be negative

One of the premises must be type A or I.

4

Both premises cannot be particular

Both premises cannot have the quantifier "some." One of the premises must be type A or E.

5

The conclusion can not be wider then the premises

A term with a quantifier of "some" in a premise cannot appear in the conclusion with a quantifier or "all."

6

The middle term cannot be in the conclusion.

Slipping the middle into the conclusion.

If the middle term is in the conclusion then the conclusion is just a restatement of one of the premises.

7

The conclusion always follows the weaker premise

If one of the premises is particular (type I or O) then conclusion must be particular. If one of the premises is negative (type E or O) then the conclusion must be negative.

8

Two affirmative premises cannot give a negative conclusion.

If both premises are of types A and I then the conclusion can not be type E or O.

9

Terms distributed in the conclusion must be distributed in the premises.

See the page on distributed terms.