Descartes on knowledge of bodies Notes for September 20

Main points

We began by putting the last part of Monday’s discussion in order. The details are covered in the notes for last time and today’s handout.

Most of our time was spent on Descartes’s twin claims about the nature of bodies (physical objects) and how we know their nature. He claims that the essence (thank you Melissa) of bodies is extension, flexibility and mutability. And he claims that we know this through the intellect or reasoning rather than sensory experience of bodies or even through the extension of our sensory experience in imagination.

I ended by trying to show how this view of bodies is connected to the idea that we’re made in the image of God. First, God doesn’t know about things through the senses. He doesn’t have to find things out by looking at them. Second, God’s knowledge of what things are and, most importantly, how they will behave is perfect. Describing bodies in geometrical terms is the first step in a mathematical physics. In other words, if we were as certain of the relationships among bodies as we are about the relationships among geometrical shapes, our understanding of the physical world would have as much certainty as our mathematics gives us. That would be pretty close to the kind of knowledge that God was thought to have

Two questions

It seems to me that we had two main questions about Descartes’s claims about bodies.

First, has he shown that bodies are necessarily flexible and mutable? Even the most skeptical of us were willing to grant him extension: all bodies have spatial dimension. But must they all be flexible and mutable?

Anna and I were uncertain about this. Tommy made a good case for Descartes’s side.

Second, has he shown that we know these things about bodies by reasoning rather than through the senses? After all, you learn that things have extension by measuring them. Using a ruler is definitely something that you do with your senses.

I think that this is how Descartes would answer that question. (This is not a quotation.)

It’s true that you use your senses to learn a particular object’s extension. But as the example of the wax showed, that particular extension is not that particular object’s essential nature. It could get bigger or smaller without becoming a different thing. You know this even though you haven’t manipulated the particular thing in various ways or even imagined all of the ways of changing its dimensions.

In fact, you know that something similar is true of bodies in general. They all have extension, but none must have the particular extension (spatial dimension or length) that it has at any particular time. You know this but you haven’t tested every single body either in fact or using your imagination.

How do you know these things about bodies? The only possible answer is that you use your reason to reach that conclusion.

Is this a good argument? It might be if there are only three possible ways of coming to believe these things about bodies: sensory experience, imagination, and reason. The elimination of the first two alternatives would leave only the third.

One of our later authors, David Hume, will propose a fourth alternative in a similar case. We draw inferences that exceed our sensory experiences, but these are driven by feelings rather than reason.** Hume will describe these feelings as working through what he calls “the imagination.” We’ll have to compare his use of that term with Descartes’s.

This page was written by Michael Green for The Image of God, ID-1, Fall 2007. It was posted September 21, 2007.
The Image of God