Question: what was the only thing descartes was sure about

title: Cogito, ergo sum
text: There are three important notes to keep in mind here . First , he claims only the certainty of his own existence from the first - person point of view -- he has not proved the existence of other minds at this point . This is something that has to be thought through by each of us for ourselves , as we follow the course of the meditations . Second , he does not say that his existence is necessary ; he says that if he thinks , then necessarily he exists ( see the instantiation principle ) . Third , this proposition `` I am , I exist '' is held true not based on a deduction ( as mentioned above ) or on empirical induction but on the clarity and self - evidence of the proposition . Descartes does not use this first certainty , the cogito , as a foundation upon which to build further knowledge ; rather , it is the firm ground upon which he can stand as he works to discover further truths As he puts it :