This is the the fourth post in my Theories of Knowledge and Reality series. Follow the link for more on the series and for links to other entries as they appear.
The first response to Descartes' particular version of skepticism didn't take very long to surface. Indeed, Descartes was its author. He wasn't a skeptic himself. His goal in writing the Meditations on First Philosophy, where his most extended treatment of skepticism is found, was to argue that our knowledge is indeed well-founded. It's just that his view that knowledge requires absolute certainty pretty much required him to present reasons for doubting before he could reconstruct the certainty of knowledge from its basis. His argument is nowadays seen to be wholly unconvincing in more than one place, but I'll sketch the general outline of it anyway so you can see how he tried to respond, because any response that might be successful would have to avoid the problems with his attempt.
Descartes picked up on Augustine's argument that there's got to be at least one thing that we know, no matter how much doubt we engagw in. Augustine was dealing with the ancient skeptics who claimed not to have any beliefs. They saw belief as a way to open yourself up to error, since nothing can be trusted as reliable. Certain things appear to be the case, and the skeptics might assume those things in some sense, but they won't believe them. Augustine argued that this very reasoning process involves thinking, and someone's got to be doing that thinking. How can you doubt something if you don't exist? I'm raising doubts, so I must exist. I think, so I must exist. The most famous formulation of this is "I think; therefore I am", which doesn't occur in Descartes' Meditations as such but is in one of his other works, and the main argument does occur in the Meditations. It's not originally his argument, though, despite the popular notion that he came up with it. It's really Augustine's argument. Descartes did make use of it, though, to begin his way back up the path to knowledge. He at least knows that he exists, because he knows he thinks.
From there, he gives an argument based on what he does when he uses his senses. He's very careful not to rely on any sense experience as evidence for something that conflicts with the skeptical hypotheses. The sense experience he has is fully consistent with the skeptical scenarios he raises, and he's aware of that. What he does is look at what he's doing with his reasoning while still being unsure if his sense experience is reliable. He reasons about the nature of things. He considers matter and mind, for instance, and notes that they have very different natures in principle. He arrives at the conclusion that they must not be the same sort of thing.
He then argues for the existence of God, starting from the very concept of God and giving an extremely poor version of an argument that Anselm had given 500 years earlier, an argument philosophers call the ontological argument. Most philosophers today are unconvinced by Anselm's version of the argument. Even Thomas Aquinas, who thought there were some excellent demonstrations of God's existence, thought Anselm's argument was fallacious. Philosophers agree that Anselm's version of it is far superior and much more philosophically interesting than Descartes' version. I happen to think the most common criticisms even of Descartes' version are wrong, but that's not relevant to what I want to say, because I still think his argument is mistaken. Virtually no one nowadays thinks Descartes established the existence of God with this argument. The reasons are technical enough that I don't want to spend time on it. I'm simply reporting what almost every philosopher agrees. Descartes did not prove the existence of God.
Once he thinks he's established God's existence, he reasons from the nature of God as a perfect being, which was part of the basis of his argument for God's existence to begine with. If God is a perfect being, then God must be morally perfect. A morally perfect being won't deceive, so God can’t be a deceiver. If our senses were unreliable on such a massive scale, God would be a deceiver, because they would be reporting massively false information. Since God created our senses, this misinformation would be atttributable to God. So we should trust our senses, Descartes concludes, because God wouldn't deceive us by giving us deceptive sense as the skeptical hypotheses he considers would require.
Most philosophers find this unsatisfying as a response to the radical skepticism Descartes begins with. The most important reason is that he didn't give a very good argument for the existence of God. There are other criticisms of his overall argument, but that's enough to show that he didn't prove that our senses are reliable. If he didn't demonstrate without possibility of doubt that God exists, then he hasn't shown anything that's inconsistent with his radically skeptical hypotheses (dreaming, evil demon, etc.), and he simply hasn't given an effective response to skepticism. Keep in mind that he doesn't think you can know something unless you can rule out all alternatives with absolute certainty.
What he wanted to do is what philosophers call an a priori argument, which is an argument based on things you can know without having investigated through your senses. It's what you approach the world with, apart from your sensory experience. An a priori argument that you can know things with your senses would indeed avoid the problem that all our evidence is consistent with both the skeptical scenario and what we ordinarily believe. The problem is that the a priori argument relies on steps that simply shouldn't convince. Descartes attempted an a priori response to skepticism, but he didn't succeed.
Yet it seems his methodological assumption requires an a priori response. Your senses are consistent with the skeptical hyothesis, so how could you possibly use your senses to show that your senses are reliable? The only other way to do it would be to have knowledge that's independent of your senses. Has anyone else come up with such an a priori argument that our senses are trustworthy? I've never seen one. I don't think any other argument for God's existence is a priori, and most people recognize that those arguments don't constitute proofs anyway (an issue I'll look at later in the series). Even if you could prove God's existence a priori, you'd also have to prove that God wouldn't be a deceiver, The a priori approach seems to be a dead-end, then. Descartes only raised these arguments so he could respond to them, but his legacy is skepticism rather than certainty, because his response to skepticism in favor of certainty was so ineffective.
So how do philosophers respond to the arguments, if his own way of responding to them has no hope? I'll look at his more immediate successors in the next few posts and then some more recent responses after that. [Update: starting here]