Why is using a criterion to justify itself considered circular reasoning?

In the context of skepticism, using a criterion to justify itself is considered circular reasoning because it assumes the validity of the very standard it seeks to establish.1

When a philosopher attempts to determine if a specific criterion is correct, they face a dilemma: if they use another criterion to justify the first, they must then justify that second criterion, leading to an infinite regress. If they instead use the criterion to justify itself, they commit a logical fallacy known as circular argument or petitio principii ("begging the question"), where the premise assumes the truth of the conclusion.12

A classic example is the Cartesian circle, where René Descartes attempted to prove that his "clear and distinct" perceptions were true by relying on the existence of God, while simultaneously using his clear and distinct perception of his reasoning to prove that God exists. Because the justification relies on the assumption it is trying to prove, it fails to provide an independent or objective foundation for knowledge.321