French philosopher René Descartes, who is regarded as the pioneer of the modern philosophy, accepted the dualism of mind and matter and objective existence of both. In his ontology, he has tackled the question of reality of the universe and the substances. Descartes, first of all, considers "self to be a reality. He, being anxious to build his metaphysics only upon what was absolutely certain, set to work as a preliminary, to doubt anything that he could make himself doubt. He succeeded in doubting the whole external world but he could not manage to doubt his own existence, for, said he, "I am really doubting, whatever else may be doubtful, the fact that I doubt is indubitable. And I could not doubt if I did not exist. He summed up the argument in his famous formula 'cogito ergo sum' (i,e., I think, therefore I am)."[1]
Secondly, Descartes has considered "God" as the reality. Apart from God, the divine substance, he recognised just two kinds of substances, two types of real entity. First, there was material subtance, or matter, and the belief that the only scientifically important characteristics of things in the physical world were their spatial characteristics, goes over, in the language of metaphysics, into the doctrine that these are their only real characteristics. Thirdly, Descartes recognised mind's or mental substances, of which the essential characteristic was thinking; and thinking itself, in its pure form at least, was conceived of as simply the intuitive grasping of self-evident axioms and their deductive consequences.[2]
In other words, according to Descartes, the essence of mind is thought and the essence of matter is extension. He made these two substances so different that interaction between them became difficult to understand, and his followers decided that there is never any effect either of mind on matter or of matter on mind.[3] Such a distinction is called as Cartesian partition. This results in the conclusion that the world, i.e., the extended things really exist. This form of realism is known as "Metaphysical Realism."[4]