cogito ergo sum
I am thinking, therefore I exist. This Latin statement, made by René Descartes, forms the basis for his rejection of metaphysical skepticism, the doctrine that nothing can be proven to have a real existence: If he (i.e., Descartes) thinks, then he must exist, and if he exists, the doctrine is wrong, leaving open the possibility of the existence of other beings and things. See Cartesian self.