Descartes is generally considered to be the first modern, as opposed to medieval, philosopher. He self-consciously began his philosophical project in opposition to the scholastic orthodoxy that was still dominant at the time. Descartes held that we should proceed by a method of doubt, that is, we should throw out all of our ideas and proceed reacquiring what knowledge we could by slowly assessing which ideas were able to withstand radical doubt. He famously starts his investigation by imagining that he does not exist and then seeks to prove his own existence from the fact of his thinking. This is the famous, "Cogito ergo Sum" or "I think, therefore I am." After establishing that he exists, Descartes goes on to prove many other things, including the existence of God.