Descartes believed that we should ask what it would mean to know about reality,
and to examine what reality meant. He claims that unless we know first whether
our belief itself is justified we can\'t know. To determine whether our beliefs
are justified, we have to be able to trace them back to a statement, belief, or
proposition that cannot be doubted. Like many other philosophers the only true
and believable facts are mathematical. But if achieved, such a proposition could
place the firm foundation on which all subsequent beliefs could be grounded; it
would guarantee that all subsequent claims based on it would be true. Descartes
was big on doubting everything. For us to distinguish this base of ultimate
truth and knowledge, of which all other knowledge can be based, Descartes
described a process. This method is to take away all confidence in which has
been taught, what the senses tell us, and what is thought is obvious, basically,
regarding all that is known by us. In order to determine whether there is
anything we can know with certainty, he says that we first have to doubt
everything. This doubting does not fully seem unreasonable. What he suggests is
that, in order to see if there is some belief that cannot be doubted, we should
temporarily believe that everything we know is questionable. Since sense
experience is sometimes deceiving, it is obvious to Descartes that there are two
operative ways of which to draw knowledge. They are intuition (A*B B*C) and
deduction (A*C). Anything else cannot be the basis for claims of knowledge. We
cannot know that what we experience through the senses is true with any
certainty. So the best thing to do is to doubt our senses. Furthermore, we
cannot be sure that we really exist, have bodies, or that experience of the
world in general can be trusted. After all, we could be dreaming the entire
thing. Next, we cannot even be sure that mathematical propositions such as 2+2=4
or that triangles always have three sides are true because some "arch
deceiver" as Descartes called it, might be deceiving us to think such a
thing. It is possible that even propositions that seem evident to us as true
might themselves be really false. But regardless if an arch deceiver deceives us
about all of our beliefs, there is one belief that we cannot be mistaken about.

And that is that we are thinking. Even doubting this is asserting it. Thinking
proves that we exist (at least as a mind or thinking thing, regardless of the
possesion of a body). The body is not an essential part of the self because we
can doubt its existence in a way that we cannot doubt the existence of the mind.

So Descartes concludes that I know one thing clearly and distinctly, namely,
that I exist because I think: "Cogito ergo sum," I think, therefore I
exist. From this starting point I can begin to note other truths that I know
clearly and distinctly, such as the principle of identity (A is A) and the
notion that things in the world are "substances." Since identity and
substance are ideas that are not based on sensation, they must be innate (that
is, they must be implicit in the very act of thinking itself). Even sensible
things (e.g., a block of wax) are knowable not based on sense experience but
intellectually, insofar as we know them to be the same things even though their
sensible appearances might change dramatically. In order to be certain that we
are not deceived when we claim to know something, Descartes must dispose of the
evil genie. This is done by proving that an all-good, all-powerful God would not
permit us to be deceived. If there is such a God, we can have knowledge. Since
the senses cannot be trusted to provide a proof that God exists, only a proof
based on the principle of the cogito ("I think, therefore I am") will
work. That proof can be summarized in the following way: I know I exist; but the
"I" who exists is obviously imperfect; otherwise I would not have
doubts about what I know in the first place. To know that I, an imperfect thing,
exist means that I already know that a perfect thing must exist in terms of
which my own existence is meaningful. I know what it means to be imperfect only
if I already know what perfection is. But I do not know perfection in virtue of
myself; therefore there must be a perfect substance (God) who exists in terms