A tautology is a statement that is true by necessity. It is true in all possible worlds. It cannot be false. Either it is raining or it is not raining. That is a tautology. It is true regardless of the weather. The statement does not tell you anything about the world. It is true by logical form.

Tautologies are common in logic. They are statements that are true under every interpretation. They are the opposite of contradictions, which are false under every interpretation. In everyday language, a tautology is a redundant statement. Free gift is a tautology. The gift is free by definition. The statement is true but uninformative.

Tautology examples

• Either it will rain or it will not rain.
• All bachelors are unmarried.
• A triangle has three sides.
• If it is true, then it is true.
• Boys will be boys.

Tautologies are not always useless. In logic, they are important. They are the laws of thought. They are the basis of proof. In rhetoric, tautologies can be used for emphasis. War is war. It is what it is. The statement is true but it conveys attitude. It says the situation is unchangeable. In philosophy, tautologies are analytic propositions. They are true by meaning. They do not add to knowledge. That is the criticism. A tautology is a closed loop. It tells you what you already know. But sometimes that is useful. It clarifies. It reminds. It emphasizes. A tautology is not a fallacy. It is a statement that is true by necessity. It is not informative. But it is certain.