Philosophers typically distinguish between a priori and a posteriori beliefs, knowledge, justification, and propositions. A belief is a priori if it is derived from reason, and it is a posteriori if it is derived from sense experience. Similarly, we would say that we know a priori that ‘a closed, n-sided figure has n interior angles’ because our knowledge is derived from reason in that we understand the concept of a closed, n-sided figure and thus know the statement is true. On the other hand, we know a posteriori that ‘Americans drive on the right’ because in justifying this belief, we appeal to sense experience; perhaps we have seen for ourselves that Americans drive on the right or we've read about it in a book or seen it in a movie.
This distinction between a priori and a posteriori beliefs is essential to the classic debate between Rationalists and Empiricists. In response to the skeptic who would argue that knowledge is not possible because beliefs can never be completely justified, the foundationalist responds that some beliefs are self-justified, and these beliefs form the foundations of all other knowledge. However, the foundationalists disagree about the nature of these basic beliefs. One sort of foundationalist, namely the Rationalist, holds that foundational beliefs are a priori whereas another sort of foundationalist, the Empiricist, holds that knowledge is ultimately justified on the basis of a posteriori beliefs.
In his classic thought experiment, what is called the Dream Argument, René Descartes (1596–1650) argues that sense experience cannot provide us with the foundations of knowledge. His reasoning is this: Knowledge requires certainty. One cannot know that some statement is true if one is not certain that it is. If one can doubt that a statement is true, that is, if one can consider the possibility that it is false, then one is not certain that it is true. For any belief formed on the basis of sense experience, he claims, one can always consider the possibility that one is dreaming and the possibility that what one believes on the basis of sense experience is false. Therefore, beliefs based on sense experience, or a posteriori beliefs, cannot form the foundations of knowledge.
Descartes would argue that while I think I know I am sitting at my computer right now, because I can consider the possibility that I am only dreaming that I am sitting here, and it is possible that I am really at home in bed, I am not certain that I am sitting here. And since I am not certain that I am sitting here, I do not know that I am. Yes, of course it seems like I am, but sense experience is not to be trusted according to Descartes. In dreams, he would argue, things seem a certain way; they seem real even when they are not. So, while it may seem to me like I am sitting here, he would say, perhaps it only seems to seem that way. Either way, since I am not certain, I do not know that I am sitting here.
The Empiricist disagrees. In his classic reply to Descartes, a lecture called ‘Certainty’, G.E. Moore (1873–1958) raises an important objection to Descartes' Dream Argument. He accuses Descartes' argument of being committed to inconsistent claims. Moore points out that at the same time Descartes wants to say that we cannot know whether we are awake or dreaming because they seem the same to us, his reason for thinking that they seem the same to us is that in the past we have awoken from dreams only to realize that while it seemed as though we were awake, we were not. But this later discovery rests on being able to know that one has awoken from a dream; however, that is just what we cannot know if Descartes' first claim is true. Moore goes on to suggest that knowledge does not require the strict sense of certainty that Descartes claims it does. Instead, he thinks that while a posteriori beliefs may be dubitable, when coupled with memories of experiences from the recent past, current sense experience provides us with beliefs that are highly probable; and these can form the foundations for knowledge.
While Descartes and Moore lived at different times, by engaging with Descartes' argument, Moore has engaged in a dialogue with Descartes. Let's listen in.
SCENE: Cambridge pub, early evening, René Descartes is sitting at the bar when his friend and colleague, G.E. Moore walks in.
MOORE: Blast! These new shoes are killing me! Barkeep, get me a pint quick!
DESCARTES: Hold on, old man, surely you don't know that your shoes are uncomfortable.
MOORE: Rubbish! What are you talking about Descartes?
DESCARTES: You can't know anything on the basis of sense experience.
MOORE: Yes I can. I know that I am standing here right now, and as soon as I get my pint I am going to sit down over there and drink it.
DESCARTES: No, you don't know that you are standing at all.
MOORE: Yes, I most certainly do.
DESCARTES: No, you don't.
MOORE: Prove that I don't!
DESCARTES: Haven't you ever dreamed that you were standing?
MOORE: Yes, of course I have.
DESCARTES: And in your dreams, did it seem like you were standing?
MOORE: Yes. So what?
DESCARTES: Does it seem like you are standing right now?
MOORE: Yes, of course, though I wish he'd hurry up with my pint so I could sit down.
DESCARTES: Maybe you are just dreaming that you are standing.
MOORE: But it seems like I am quite awake. So I must be awake.
DESCARTES: But doesn't it seem like that in dreams? Don't dreams sometimes seem real?
MOORE: Not always. Sometimes a dream seems like a dream.
DESCARTES: But sometimes dreams seem quite real, don't they?
MOORE: Yes, I admit they do. Though I cannot possibly imagine why I would dream that I was wearing such uncomfortable shoes and waiting this long to get my beer!
DESCARTES: Forget your beer. Listen to this: You admit there is some possibility that it only seems like you are standing, and that you are not actually standing.
MOORE: Yes, okay. I admit it. There is a very slight possibility that this is a dream, or maybe a nightmare.
DESCARTES: So, you are not certain that you are standing. Since knowledge requires certainty, you do not know that you are standing because you are not certain.
MOORE: But wait, Descartes, I believe you have made an error in your reasoning.
DESCARTES: Impossible!
MOORE: You say that if I do not know I am not dreaming, then I cannot know that I am standing here right now.
DESCARTES: Exactement!
MOORE: This is just to say that if I know I am standing, then I must know I am not dreaming.
DESCARTES: Of course, that is right. Either way, it is true.
MOORE: However, you say that I cannot know that I am not dreaming.
DESCARTES: Oui, I do!
MOORE: But your reason for thinking that I cannot know that I am not dreaming is that while it seems to me that I am standing, it would seem exactly the same way if I were dreaming; so I cannot be certain now that I am not dreaming.
DESCARTES: YES! I think you've got it!
MOORE: But, Monsieur Descartes, how do you know that how things seem when you are awake is how they seem when you are dreaming? Or, if you like, how do you know that how things seem when you are dreaming is how they seem when you are awake?
DESCARTES: Well, in the past, I have had the experience of waking up and thinking, ‘oh that dream felt so real’; it seemed to me that I was really standing even though I was not.
MOORE: AH HA! So now I've got you!
DESCARTES: What are you talking about, fool?
MOORE: In the past, you were able to distinguish real experience from dream experience in order to determine that your dreamt experiences seemed just like your wakeful experiences!
DESCARTES: Yes, so?
MOORE: But you argue that you cannot know that you are not dreaming, so how did you know that you were not in a dream when you compared how things felt in your dream to how things felt when you thought you were awake?
DESCARTES: Twizzle sticks!
MOORE: Again, Monsieur Descartes, I assert that I know that I am standing, and I have been standing here entirely too long. Barkeeper!
DESCARTES: On what grounds do you claim to know this, Professor Moore?
MOORE: It seems that I am standing! And, on top of that, it has seemed that way to me for the last few minutes!
DESCARTES: But, you could be dreaming and it could be false that you are standing.
MOORE: Indeed. While there is some small possibility that it is false that I am standing, in light of my present and past sense experience, it is highly likely that I am standing.
DESCARTES: But it is not necessarily true!
MOORE: Of course, it is not. But knowledge does not require the strict type of justification you think it does, Descartes. The justification I have based on my sense experience is good enough for knowledge! Now, here's my pint. I have had quite enough of this standing here. Let's sit down, Monsieur Descartes. You, my friend, are buying the next round.
DESCARTES: Oh drat!
