After proving with the Evil Demon thought experiment that his thoughts were the only things that he could be certain of, Descartes extrapolated the concept against the Monists. If Descartes could only be sure of the existence of his mind, how could his mind and body be one? It did not make sense, which led Descartes to conclude that the mind and body were fundamentally distinct.

In addition, Descartes argued that if the body could be divided but the mind could not, as far as we know, they could not be made of the same substance, and therefore had to be distinct. If the body was physical and material, the mind (which controlled thoughts and consciousness) had to be both immaterial and non-physical. While separate, Descartes still suggested that the mind and body closely interacted with each other.