  As mentioned in 3.1, Descartes held that any act of judgment, such as the affirmation “I think, therefore I am,” involves both the intellect and will. The intellect perceives or represents the content of the judgment; the will affirms or denies that content. In the face of genuine clarity and distinctness, “a great light in the intellect” is followed by “a great inclination of the will” (7:59). The inclination of the will is so strong that it amounts to compulsion; we cannot help but so affirm.  Descartes thus makes unshakable conviction the criterion. Can't someone be unshakable in their conviction merely because they are stubborn? Assuredly so. But Descartes is talking about a conviction that remains unshakable in face of serious and well-thought out challenges (7:22). To be immune from doubt does not mean simply that you do not doubt a proposition, or even that it resists a momentary attempt to doubt; the real criterion for truth is that the content of a proposition is so clearly perceived that the will is drawn to it in such a way that the will's affirmation cannot be shaken even by the systematic and sustained doubts of the Meditations. Perhaps because the process for achieving knowledge of fundamental truths requires sustained, systematic doubt, Descartes indicates that such doubt should be undertaken only once in the course of a life (7:18; 3:695).