Descartes used skepticism to displace the senses. We think of them as our chief source of knowledge but the First Meditation showed that we are mistaken. We cannot know anything on the basis of the senses alone.
Descartes himself was not a skeptic. He thought that reason was our most fundamental source of knowledge. We can use reason to understand the true nature of bodies, why God must exist, and why we can trust the senses. We saw some of his arguments for those conclusions in the Third Meditation. (There are three more meditations that we will not read. As you might imagine, they all make arguments.)
Or a clear and distinct perception? These are terms that Descartes liked to use to mark things he thought he clearly knew. To know something by “the natural light” is genuinely to know it. Merely seeing something in natural light, by contrast, is not enough. But is there anything to this beyond some brave sounding labeling? “This I know! That, I’m not so sure about. What’s the difference between this and that? The natural light!”
Well, I think there is something to it. I introduced my interpretation of these phrases in the course of revisiting a question we had from the First Meditation: can you be deceived about simple statements of mathematics? Descartes was pulled in two ways on this question.
On the one hand, it’s hard to see where the space for error might be. I don’t know whether my visual impressions of trees are caused by real trees. There’s space between what is going on in my head and what is happening outside my eyeballs for all sorts of trickery. Maybe they’re fakes, maybe there’s a demon messing with me, or maybe a dream is creating visual impressions while my eyes are shut. But where’s the room for error in 1=1? If you understand what ‘1’ and ‘=’ mean, you can’t be wrong.
On the other hand, who knows what an omnipotent power can do?
I think that, in the end, Descartes took the first option: there are some kinds of reasoning that are immune from error. A demon may trick me when it comes to an extended chain of reasoning but not when it’s obvious. At the very least, Descartes needed to believe something like this. If he didn’t, the reasoning in the Third (and subsequent) Meditations wouldn’t be any better than the evidence from the senses that he found inadequate in the First Meditation.
As the handout indicates, there are two arguments for God’s existence. One seeks to show that only God could cause the idea of God. Since many people have this idea, God must exist to have caused it. The other seeks to show that only God could preserve things from one moment to the next. We talked about the first; I put off the second for our discussion of Hume on the necessary connection between cause and effect.** This is a version of the doctrine of ‘occasionalism,’ meaning that so-called natural causes are merely occasions for God to exercise his power. Descartes thought this had to happen every instant, since matter cannot preserve itself on its own.
Suppose we grant these arguments. What does that tell us about our knowledge? Not much. We need an additional premise that God would not allow us to be deceived. Descartes supplies that at the very end in a paragraph that starts with assertions that we are made in God’s image and concludes that God follows a moral code pretty much like ours. That’s why He wouldn’t allow us to be massively deceived.
The details of the argument are very difficult to follow. But there are a couple of challenges to its conclusions that seem clear enough even if the details are granted. Both are on the handout.
Arnauld charged that the argument was circular. Descartes used reasoning to show that we have good reason to believe in God and our beliefs about God to show that our reasoning is reliable. So if you asked “why can we rely on our reasoning,” the answer would be “we know that God exists and that He would not allow us to be massively deceived.” But if you asked why we know that God exists, etc., the answer would be that our reasoning shows these things. Each answer depends on the other, such that two simple questions about them will lead you in a circle.
Bayle charged that the conclusion about knowledge does not follow. Descartes himself had to admit that God allows us to be pretty deeply confused about bodies. We think that bodies have colors, tastes, smells, and all the other qualities that we perceive through the senses. But, according to Descartes, reason shows that bodies are really just extension and nothing more. Needless to say, if he’s right, we all believe mostly false things about bodies. Bayle drew the obvious conclusion: if God has no problem with letting us be so wrong about bodies, why is it obvious that he wouldn’t let us be deceived about the existence of the external world?
I have an idea of why Descartes didn’t consider that possibility. He believed that we were made in the image of God and this means that we have the ability to understand the world. Before the Fall, we (that is, Adam) did understand the world. We lost that ability as a consequence of sin. But if we concentrate hard enough, we can get it back. That was an idea behind Descartes’s philosophy. It is also the idea that Hume sought to knock over.