Before applying this method to the other sciences, Descartes thought it well to find some philosophical foundations for his method.

Analysis

If we were to identify a starting point for modern philosophy, November 10, 1619 would be as good a date as any. We might pinpoint precisely the moment that Descartes resolved to cast all his former opinions into doubt. This process of methodological doubt is central to Descartes, and indeed to most of modern philosophy. The results Descartes achieves by employing this method of doubt are discussed in Part Four of the Discourse, so we will comment on his method in greater detail there.

It is important, of course, that Descartes does not simply scrap everything he knows, or else he would have no guidance in rebuilding his knowledge. The four rules he lays out are meant as guidelines, so that he will be able to rely on them, and not on unnoticed prejudices. Descartes had initially collected twenty-one rules entitled Rules for the Direction of Our Native Intelligence in 1628, but left the manuscript unpublished. The four rules we find here can be read as a major abbreviation of that effort. Essentially, they demand that an inquiry proceed slowly and carefully, starting with basic, simple, self-evident truths, building toward more complex and less evident propositions.

Descartes assumes a certain kind of theory of knowledge that was pretty much unquestioned in his day. In modern philosophical language, we call this a foundationalist epistemology. It sees knowledge as built up from simple, self-evident propositions, to higher and more complex knowledge. The theory states that if we were to analyze any complex proposition, we could break it down into increasingly smaller, simpler pieces until we were left with simple, non-analyzable propositions. These basic propositions would be either self-evidently true or self-evidently false. If they were all true, then we would know that the original complex proposition was also true. Of course, there are different variations of foundationalist epistemology; for example, the epistemology will shift depending on how the analysis is supposed to take place or on what the basic propositions are supposed to look like. But the general idea can be applied to Descartes easily. Knowledge is built up like a skyscraper, with the higher, complex knowledge built on simple, sturdy foundations.

This is just one of a number of theories of knowledge that are batted about these days. Another theory that will come into play later in the Discourse is a coherentist epistemology, one that states that knowledge is more like a circle than a skyscraper. According to this theory, there is no foundational knowledge that is more basic than other knowledge. All knowledge fits together in such a way that it is internally coherent, but there is no fundamental self-evident proposition that is itself beyond doubt and that justifies all the other propositions. A statement is true because it is consistent with everything else we know to be true, not because it can be analyzed into simple parts.

The reason that a foundationalist epistemology seems natural to Descartes at this point is that this is the epistemology that philosophy had inherited from Aristotle. As we have noted already in other sections of this SparkNote, Aristotelian scientific method works according to a system of syllogism and demonstration, where complex truths are logically deduced from simpler ones. This method implies a theory of knowledge according to which complex truths are built upon simpler ones that serve as an unquestioned bedrock of knowledge.