Descartes' Methodic Doubt
René Descartes (1596-1650) is an example of a rationalist. According to Descartes, before we can describe the nature of reality (as is done in metaphysics) or say what it means for something to be or exist (which is the focus of ontology), we must first consider what we mean when we say we know what reality, being, or existence is. He suggests that it is pointless to claim that something is real or exists unless we first know how such a claim could be known as a justified true belief. But to say that our beliefs are justified, we have to be able to base them ultimately on a belief that is itself indubitable. Such a belief could then provide a firm foundation on which all subsequent beliefs are grounded and could thus be known as true. This way of thinking about knowledge is called foundationalism.

In his Meditations on First Philosophy (1641), Descartes indicates how we are able to guarantee our beliefs about reality by limiting what we believe to what is indubitable or is based on what is indubitable. That involves him in a series of six "meditations" (of which we will focus on only the first two) about the proper method of philosophical reflection and the conclusions that can be drawn from using that method. Throughout these Meditations Descartes insists that (1) we should claim to know only that for which we have justification, (2) we cannot appeal to anything outside of our ideas for such justification, and (3) we judge our ideas using a method that guarantees that our ideas are correct.

In the first Meditation Descartes argues that our ordinary experience of the world cannot provide the kind of guaranteed foundation on which all other knowledge can be based. We are often disappointed to learn that what we have been taught are merely prejudices, or that what our senses tell us is incorrect. That should make us wonder about whether