Descartes set a standard that our beliefs have to pass if they are to count as genuine knowledge. Then he argued that what we believe on the basis of the senses cannot meet the standard. Consequently, he concluded, we do not know anything on the basis of our senses.
Descartes himself did not rest with this conclusion. He thought it showed that we could not know anything on the basis of the senses alone. Reason did the heavy lifting for him. But I’ll leave that for the next installment.
Descartes subjects our beliefs to doubt. What does that mean? It does not mean that he gives us reason for believing things that are incompatible with the beliefs we currently have. He does not maintain that he has evidence that he is asleep or that there is an evil demon messing with his head.
The method of doubt is more indirect. It involves two steps:
For example, suppose I believe there is a tree outside my window because I see a tree outside my window. According to the assumption, I would have to know that I’m not dreaming about seeing a tree in order to know that there is a tree because I see a tree. If I were dreaming, then I would not be seeing a tree (or anything at all) and so my seeing a tree could not be the basis for my knowing that there is a tree.
The argument for the claim is that I cannot distinguish between the truth and falsity of the scenarios. For instance, I cannot distinguish between seeing the world and having an exceptionally realistic dream of seeing the world. If I cannot tell the difference, then I cannot know whether I am seeing things or having an exceptionally realistic dream of seeing them.
On the one hand, it seems that it is. We don’t normally require that people be able to eliminate all incompatible possibilities in order to claim to know things. It would be extraordinarily tedious of you to insist on that standard in everyday conversation.
On the other hand, Descartes’s standard seems to be just a general application of a perfectly ordinary, sensible standard for knowledge. This was the point of the example of the yellow bird.
Suppose I see a yellow bird and conclude that I know it is a goldfinch. Suppose that I do not know that it is not some other kind of yellow bird, such as a canary or a yellow-tailed Oriole. Most of us will conclude that I don’t really know it is goldfinch. Descartes’s assumption appears to explain why: I don’t know that the various possibilities incompatible with my knowing it is a goldfinch are not true.
What is disconcerting about Descartes’s argument is that it takes this perfectly normal standard for knowledge and shows that it leads to the quite extraordinary conclusion that we know nothing at all on the basis of the senses.