Sceptical argument 3: The dream argument.

I ought not to trust my sense ideas because I could be dreaming and not know it. The sense ideas that I have may not be caused by anything in the external world, they may just be a dream.

This argument is more powerful than the crazy argument because it doesn’t break the rules. We have all had the experience of having a very realistic dream, while we have not all had the experience of being crazy.

Realist response: The painting analogy

Even if you are dreaming, the ultimate causes of the ideas in your dreams are your sense experiences of the real world. Therefore even if you are dreaming, you can still trust that the sense ideas in your dream are caused by the real world. To show this, Descartes makes an analogy between a dream and a painting.

Imagine a realistic painting of something that doesn’t exist, like a unicorn. Even though there are no unicorns in the world, the ideas that the painter expresses when he paints a unicorn are provided by his sense experiences of a horse and of a goat. He just changed them a little bit when he made the painting. Just because it is a painting of something that isn’t real, it doesn’t mean that the ideas in the painting weren’t caused by ideas in the real world.

By analogy, even though we may be dreaming, the sense ideas in our dream had to be caused by our previous sense experiences of the real world.

Sceptical counter attack: Abstract art

What about a painting that is completely abstract and is not based on any sense ideas at all. Couldn’t our dreams belike this sort of painting? Just like a painting can be brought into existence without being a painting based on anything that we have experienced, so could our dreams come into existence without being based on a sense idea. IE there could be a dream the content of which is not based on anything that we have sense experience of.

Realist response:

Even a piece of abstract art has to be based on colors and shapes. The painting analogy doesn’t hold in terms of the content of the painting, but it holds in terms of what the painting is made up of. Even though we may be dreaming, the knowledge of things like colour and shape and number is still knowledge that we can trust.

At this point, the sceptic concedes.

The realist only has a limited victory here because even though we don’t have to give up our knowledge of things like shape, we do have to give up our knowledge of sense ideas.

Descartes the sceptic will go on to use the evil demon argument to make us doubt even our knowledge of things like shape and geometry.