The following rules must be observed in order to form a valid categorical syllogism:

Rule-1. A valid categorical syllogism will have three and only three unambiguous categorical terms.

The use of exactly three categorical terms is part of the definition of a categorical syllogism, and we saw earlier that the use of an ambiguous term in more than one of its senses amounts to the use of two distinct terms. In categorical syllogisms, using more than three terms commits the fallacy of four terms.

 Fallacy: Four terms 

Example:

 Power tends to corrupt Knowledge is power Knowledge tends to corrupt 

Justification: This syllogism appears to have only three terms, but there are really four since one of them, the middle term “power” is used in different senses in the two premises. To reveal the argument’s invalidity we need only note that the word “power” in the first premise means “ the possession of control or command over people,” whereas the word “power” in the second premise means “the ability to control things.

Rule-2. In a valid categorical syllogism the middle term must be distributed in at least one of the premises.

In order to effectively establish the presence of a genuine connection between the major and minor terms, the premises of a syllogism must provide some information about the entire class designated by the middle term. If the middle term were undistributed in both premises, then the two portions of the designated class of which they speak might be completely unrelated to each other. Syllogisms that violate this rule are said to commit the fallacy of the undistributed middle.

Fallacy: Undistributed middle

Example:

 All sharks are fish All salmon are fish All salmon are sharks