Descartes' overall project in Meditations on First Philosophy is epistemological, meaning that he is interested in how (or even whether) we know what we know. He engages in a mental exercise in which he subjects everything he could possibly know to doubt. He claims that we cannot really know the existence of our own bodies, or really anything having to do with spiritual reality. He comes to the conclusion that he can know one thing for certain. He exists, because he has the ability to question whether he exists. As he puts it: "I think, therefore I am (cogito, ergo sum.)" Having established at least one thing we can know, Descartes proceeds deductively to another conclusion. He surmises that because he exists, and because he can conceive of God, then God must exist. We imagine God, who is perfect, against ourselves, who are imperfect. Only God could have caused one to think this way in the first place. Numerous philosophers had tried to "prove" the existence of God, and Descartes' efforts to do so are primarily important in his overall aim, which was to provide a system by which we could deduce philosophical ideas from "first principles." His method was clear, involving proceeding from simple to more complex ideas through the synthesis of established principles.