The full title of Rene Descartes's little book, written in 1640–41, is Meditations on First Philosophy. The word first is notable. If you sat down to philosophize on several meanings, sitting before the fireplace as Descartes did on separate evenings, what would come first in order? Wouldn't you start with a survey of the things you thought you could know? What could you know with certainty? That might be the place to start. In these six meditations, which constitute Descartes's best-known work, he is searching for what is first — some proposition that cannot be doubted so he can use it as a foundation for knowledge.

René Descartes takes us step by step on this intellectual journey, this quest for certainty. This is the same Descartes who wrote in his Discourses:

From my childhood I have been nourished upon letters, and because I was persuaded that by their means one could acquire a clear and certain knowledge of all that is useful in life, I was extremely eager to learn them. But as soon as I had completed the course of study at the end of which one is normally admitted to the ranks of the learned, I completely changed my opinion. For I found myself beset by so many doubts and errors that I came to think I had gained nothing from my attempts to become educated but increasing recognition of my ignorance.

So his starting point is reality and what can be known about it. Here, Descartes uses a metaphysical approach, asking “What is out there? What kinds of things do I know?” As soon as you ask those questions, you're doing epistemology, too. For the following question is, “How do you know the ordinary things you think you know?”

For Descartes, the skeptical knife cuts even deeper than questions in philosophy.

Even sciences — like physics, anatomy, and chemistry, to name just a few — are ultimately founded on observations and more observations, all made by your senses.

But of course it must be different with mathematics. Since Descartes thinks these studies don't depend for their conclusions upon really existent material objects, your doubts about empirical science don't apply to them. So when you read the stock page and it mentions losses, gains, and dividends, you can trust those reliable indexes, no? And when you read about the scores in yesterday's sports pages, aren't they also certain?

But here Descartes uncovers a fresh reason for doubt. Suppose there is a “malicious demon” of the “utmost power and cunning,” one who has employed all his energies to deceive you, he says in the first meditation. Could it be that the reality you claim to observe is nothing but a dream that such an all-powerful being has created in your mind? Surely this is a possibility. Thus even sciences like geometry and calculus are cast into doubt.