Descartes 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.

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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 of which my own imperfect existence is intelligible. No perfect (all-good, all-powerful) being would deceive us into thinking that we know something with certainty when, in fact, we are mistaken about it. So if there is a God, then no arch deceiver could exist who tricks us regarding clear and distinct knowledge (such as mathematical reasoning). We have a “great inclination” to believe that there are physical objects that are external to the mind.

But since only those objects known in terms of mathematical properties–not those imagined by use of the senses–can be known clearly and distinctly, the only knowledge we can have of such objects is in terms of mathematical, quantifiable physics. The only real knowledge we can have, then, is of things understood as functions of laws of physics. The objects we see are not the objects we know, because what we know is intelligible only in terms of the clarity and precision of the formulae of physics. Information provided by the senses cannot therefore be the basis of knowledge.