 Descartes sometimes speaks of things that have possible existence, in addition to speaking of things as having actual existence. He also speaks of eternal and necessary truths that are the product of God’s free and wholly unconstrained activity. One of the interpretive projects that is inspired by Descartes’ sometimes provocative claims about possibility and necessity is the construction of a general Cartesian theory of modality. Any such theory would of course need to be sensitive to all of the claims that Descartes makes that bear on possibility and necessity, but it would also need to understand and interpret those claims in the light of the terminology and commitments of his larger metaphysical system.