 Descartes reveals his ontology implicitly in the Meditations, more formally in the Replies, and in textbook fashion in the Principles. The main metaphysical results that describe the nature of reality assert the existence of three substances, each characterized by an essence. The first and primary substance is God, whose essence is perfection. In fact, God is the only true substance, that is, the only being that is capable of existing on its own. The other two substances, mind and matter, are created by God and can only exist through his ongoing act of preservation or conservation, called God's “concurrence” (Princ. I.51).