Tautology

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Logical Tautology

In logic a tautology when a conclusion implies only its premise, and the premise implies only its conclusion. That is, the difference between the conclusion and the premise is merely formal, and no new information is implied. (See Wolfram research's definition) Logical tautologies are true regardless of the truth values of the individual statements. For example:

B is A or B is not-A

A logical tautology is neither good nor bad, but is a mathematical description of the truth value of a particular statement not implying any particular state of truth values of its component parts.

Rhetorical Tautology

Generally, when a word is defined with another which is an exact synonym, which, in turn, is defined with the first word. Often used to prevent refutation by counter-example.

For example the statement:

"All members of the House Republican Caucus who were serving in January of 2003 voted for the resolution of organization."

Is a tautology, because the definition of the House Republican Caucus consists of those people who voted for the resolution.

Often a tautology is used in combination with a logical fallacy, commonly the fallacy is one of equivocation, using a word or phrase which has two definitions. For example:

"Therefore all Republicans support that resolution today."

From the above would be a fallacy of equivocation, because while the word "Republicans" is often used a short hand for "members of the Republican House Caucus", the word can also mean other groups of people.