What van Meegeren had truly mastered was the art of poisoning the wells. In the context of reasoning, the technique stems from the following fact: We do not assess whether a claim is reasonable simply by thinking about that claim in isolation. Usually, we relate the claim to a body of beliefs we already hold. Relative to that corpus of accepted beliefs, we decide whether the new claim is a reasonable one to make. Given this feature of reasoning, one sees how our very capacity to assess claims can be radically undermined if one poisons the body of beliefs relative to which we normally judge matters. If we don’t know which of our background beliefs to trust, then how can we appeal to them in deciding whether to believe a new claim? And since that is usually the only way of deciding these matters, such a poisoning of the wells of belief leaves us powerless to make any further decisions about what to believe.

Behind every great philosophical skeptic there lies a well poisoner. And there is no more bravura performance in the history of philosophy than René Descartes’s “Meditations on First Philosophy.” He begins by trying to show that none of his beliefs about the everyday world around him can be maintained. These include such obvious beliefs as that he has two hands, that there are other people moving around him, that mountains do not fly, and so on. How does he do this? He poisons the wells! What are the wells in this case, what is the source of our confidence in such beliefs?

Our confidence in these beliefs rests on our confidence that our sensory apparatus, our eyes and ears, delivers accurate information about the natural world. What Descartes proceeds to do is sow doubt about the reliability of our senses through a series of ingenious arguments, which includes famously raising the possibility that we are dreaming. He proceeds like the enemy army that sees the inefficiency in denying individual households access to the communal water supply and instead opts for the expedient of well poisoning, which in one fell swoop achieves the general goal. In fact, Descartes eventually does such a thorough job of poisoning the wells of belief that he is left with virtually nothing he can believe at all.

Descartes as it turns out is not himself a skeptic, and he spends the remainder of the “Meditations” trying to pull himself out of the skeptical hole he has dug for himself. He does so to his satisfaction. But it is fair to say that he did not convince most of his readers, and there is still a lively debate as to how, if at all, it is possible to regain one’s footing once the sources of rational belief have been radically called into question.