[Originally published as ch. 1 in The Significance of Philosophical Skepticism]
In this chapter, Stroud examines the Cartesian argument for skepticism about the external world. Reflecting on our knowledge is a common, everyday task, e.g. when we examine if what we know about the common cold can be true. Descartes did this common, everyday task, except he did so over all his knowledge. He did this by finding a common source to most of his beliefs: the senses. He found the best possible case that one could be in to get knowledge through the senses: sitting in front of his fire, paper in hand. If this case is truly representative of how we get knowledge (or, more accurately, how we think we get knowledge) of the external world, then Descartes’ findings about this case will be representative of sense-knowledge. He found that he couldn’t distinguish reality from dreaming, and you don’t *know* things on account of your dreams; so he couldn’t *know* that he was sitting by the fire.
Descartes assumes: (1) you can’t know something on account of your dreams; (2) to know X through your senses, you’d have to know you weren’t dreaming when you thought you were learning X through your senses; and (3) you can’t distinguish dreaming from being awake.
The first assumption is widely regarded as correct; the way we use “know” in everyday life excludes us from knowing something on account of our dreams. The second assumption is more difficult to discuss. To know X, we don’t have to know the truth of every proposition that must be true if X is true; if this was true, we’d have to know an indeterminate number of propositions. But we do have to know the falsity of some things that contradict what we know, and we have to know the falsity of things that undermine our evidence for believing what we believe. But it’s not clear how much we have to know in order to know X. Descartes idea that we have to know we aren’t dreaming to know X has been intuitively forceful throughout history, however, so it’s at least plausible. Thirdly, if you accept that you must know that you aren’t dreaming to know anything, then you can never have knowledge of anything. Because, how can you know the evidence/information you use as the basis of your belief that you aren’t dreaming is reliable, if you can’t *know* anything unless you know you aren’t dreaming? This quickly turns into an infinite regress, because to use evidence to prove the reliability of the evidence you are using to prove you aren’t dreaming would mean that you’d have to know that this secondary information is reliable. And if you aren’t certain about the evidence/information you use to prove you aren’t dreaming, how can you be certain you aren’t dreaming?
So, prima facie, Descartes’ arguments are compelling. We are left behind a veil that separates us from reality. We have lost knowledge of chairs, tables, rocks, etc. We’ve even lost knowledge that other people exist. We can’t dismiss Descartes’ conclusion on the basis that it is some kind of philosophical knowledge, not the normal knowledge people care about, because Descartes reflected upon knowledge in the manner that people reflect upon knowledge everyday.
5 KEY POINTS
- Dreaming and knowing aren’t incompatible; you can know X to be true, and dream X, but by dreaming you don’t *thereby* know X.
- It’s not the fact that you are dreaming that keeps you from having knowledge, it’s merely the fact that you *might* be dreaming and you don’t *know* you aren’t.
- Descartes assumes that anything that can occur in real life can occur in your dreams.
- To establish that he’s not dreaming he would need something more than just experience or information alone that shows he’s not dreaming, he would also need to know that these experiences or that that information is reliable.
- “A possible deficiency in the basis of my belief can interfere with my knowledge without itself rendering false the very thing I believe.”
