That the cogito is the first truth discovered amidst the universal doubt achieved in Descartes' Meditation I is not what ensures its primacy as a metaphysical principle, i.e., its epistemic priority over scientific knowledge. This primacy depends rather on two conditions which Descartes sets forth in a letter to Clerselier. Meditation II contains the demonstration of the cogito's conformity to each of these conditions, and hence of its primacy over knowledge of nature. It seems, however, that the cogito enjoys this primacy over truths of mathematics and of metaphysics (including the principle of non-contradiction) as well.