By the end of his Second Meditation, 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 my self; 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 evil genie 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.

Certitude is thus grounded in the knowledge of the self, which is itself intelligible only if there is a God who guarantees that we are not deceived about what we know of the world clearly and distinctly (i.e., mathematically). By appeal to reason alone, we are able to know: this is the main message of rationalism.

Objections to Rationalism:

• There is no agreement among philosophers or cultures about so-called self-evident ideas. Supposedly self-evident ideas have often been rejected at later times in history.
• Self-evident ideas or a priori propositions provide no knowledge about the actual world. Though sense experience may not be certain, it provides us with information which is as reliable as we need.
• The fact that a belief is not absolutely certain should not disqualify it for knowledge. Why not say that something is known as long as there is no good reason to doubt it? Of course, that might mean that occasionally we would have to admit that what we thought we knew was something that we really didn't know. So what?