Since everything he had previously accepted as being absolutely true and
assured had been learned from or through his senses, which had sometimes
played him false, Rene Descartes believes to be certain only that which can be
indubitably proved to be so. Only then can something be established as firm and
constant in the sciences. Even arithmetic and geometry are open to doubt since
it is possible that God or an evil demon could be deceiving him into perceiving
them incorrectly. Descartes comes to the conclusion that he thinks and therefore he exists. This belief is proved to be a fact by his ability to express it or conceive of it in his mind.
Although we do make certain assumptions about the nature of the real world, such as its being continuous, bound by physics and linear, there is no way to prove these things as completely indubitable in the way Descartes intends. We cannot be certain that the real world actually exists the way that we perceive it to. But while it is possible to contemplate the nature of our existence the fact of it is inescapable.
Descartes believes himself to essentially be a mind, a thing that thinks and seeks to know itself. He maintains that while he is the being who apprehends and knows things by the sense organs, such as seeing light, hearing noise, and feeling heat, these things may indeed still be false. His body exists only in his
perceptions of it through the mind. But while the mind can exist without the body, the body cannot exist ...