Allegedly circular reasoning used by René Descartes to show that whatever he perceives "clearly and distinctly" is true.
Descartes argues that clear and distinct perception is a guarantor of truth because God, who is not a deceiver, would not allow Descartes to be mistaken about that which he clearly and distinctly perceives. The argument relies on Descartes's earlier proof of the existence of God. But Descartes cannot know that this proof does not contain an error unless he assumes that his clear and distinct perception of the steps of his reasoning guarantees that the proof is correct. Thus the criterion of clear and distinct perception depends on the assumption that God exists, which in turn depends on the criterion of clear and distinct perception. See also cogito, ergo sum .