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 all the other things that we think are obvious might likewise be mistaken. In order to test whether what we think we know is truly correct, Descartes suggests that we adopt a method that will avoid error by tracing what we know back to a firm foundation of indubitable beliefs.
Of course, it is possible that there are no absolutely unshakeable truths. It is also possible that we might discover that our prejudices cannot be removed or that beliefs we think are ultimate foundations for all our other beliefs are not really ultimate at all. The point of our meditations is to challenge those beliefs, even if we have held them for a long time. And that self-critique will take a real effort.
In order to determine whether there is anything we can know with certainty, Descartes says that we first have to doubt everything we know. Such a radical doubt might not seem reasonable, and Descartes certainly does not mean that we really should doubt everything. What he suggests, though, is that in order to see if there is some belief that cannot be doubted, we should temporarily pretend that everything we know is questionable. This pretence is what is called a hypothetical doubt. To make sure that we take the pretence seriously, Descartes suggests that there might be good arguments to think that such doubting is justified (and thus more than simply something we should pretend to do). His arguments fall into two categories: those aimed against our sense experiences and our supposition that we can distinguish between being awake and dreaming, and those aimed against our reasoning abilities themselves.
Since sense experience is sometimes deceiving, it is obvious to Descartes that a posteriori claims (e.g., that this milk tastes sour or that suit is dark blue) cannot be the basis for claims of knowledge. We do not know that what we experience through our senses is true; at least, we are not certain of it. And we cannot tell when our senses are correctly reporting the way things really are and when they are not. So the best thing to do is to doubt whether any knowledge can be based on our sense experiences.
Furthermore, how do we know that we are not dreaming some particular experience we have, or that we are not dreaming all of our experience of the world? When we dream we imagine things happening often with the same sense of reality as we do when we are supposedly awake. Just as a person who has an amputated limb has real sensations and feels real pains in a hand or a foot that no longer exists, we sense that we have a body and interact with other bodies. But isn't it possible that we are dreaming that there are things that exist apart from our thinking or dreaming about them?
Note, in his dreaming argument, Descartes is not saying that we are merely dreaming all that we experience; nor is he saying that we cannot distinguish dreaming from being awake. His point is that we cannot be sure that what we experience as being real in the world is actually real.
Next, we cannot be sure that our reasoning abilities can be trusted: we cannot be certain that 2+3 really equals 5, that triangles always have three sides, that a whole is always greater than any one of its parts, or that if A=B and B=C, then A=C, because some evil power (though not God, who is all-good) might be deceiving us to think such things when it is possible that such propositions and the judgments based on them that seem obviously true might really be false. Meditation One ends in this doubt-filled state, prompting Descartes to wonder if anything can be known with the kind of certainty that he had hoped to use as the basis for all claims of knowledge.
Objections to foundationalism and Descartes' method of doubt. Critics have raised a number of objections to Descartes' way of setting up the problem of knowledge. For example:
But even if an evil genie deceives us about all other beliefs, there is one belief that we cannot be mistaken about, and that is that we are thinking/doubting. Even to doubt this is to affirm that we are thinking. And since thinking cannot occur without there being something that does the thinking (namely, me), "I" must be a thinking thing. Thinking proves that we exist, at least during those times that we think. And when we think, we are thinking things or minds, regardless of whether we have bodies. In fact, the body I experience as my own need not be an essential part of my self because I can doubt its existence in a way that I cannot doubt the existence of my mind.
A common objection at this point concerns whether Descartes is justified in saying that, just because thinking occurs, we can conclude that there is a thing that does the thinking. For Descartes, the "I think" seems to imply that there is a subject engaged in the activity of thinking. But (the objection goes) to conclude that there really is a subject who thinks is to be bewitched by the grammatical structure of the sentence. In response to this objection, Descartes implies that no action (e.g., thinking) can occur without something or someone doing the action. That someone is the self who does the thinking.