  Descartes was dead long before Leibniz articulated this criticism but it was familiar to him from the Second Set of Objectors (Marin Mersenne et al.) (AT 7:127; CSM 2:91). He replies by appealing once again to the principle of clear and distinct perception, which states that if something is contained in the clear and distinct idea of something then it is not only possible but also true of that thing in reality. (Descartes might have said that if something is conceivable then it is possible, and a being having all perfections is conceivable, but he has an even stronger principle at his disposal in the rule for truth.) In effect, Descartes thinks he has already satisfied Mersenne and Leibniz’s extra condition.  But Mersenne’s version of the objection goes further, urging that in order to know with certainty that God’s nature is possible, one must have an adequate idea that encompasses all of the divine attributes and the relations between them (ibid.) — something that Descartes denies that we have. Descartes responds to this criticism as follows: