Circular Argument

This is also called Circulus in probando.

In a circular argument, you have two propositions P and Q, but these are used in a circular fashion as follows:

Circular arguments are logically valid, because there is a logical equivalence between P and Q, but it's a fallacy just the same because it doesn't prove anything: If P is false, Q is false and if P is true, Q is true.

Examples

The bible is the word of the Judeo-Christian god, which says that the god exists, who in turn wrote the bible, which is the word of the god.

Weaknessess

Circular logic is usually presented without evidence to prove either premise.

You can feel justified in asking whether a circular argument is put forth to avoid presenting evidence for the premises.