Kant’s argument is that we have no basis to assume that our perception of reality ever resembles reality itself. Our experience of things can never penetrate to things as they really are. That reality remains permanently hidden to us. To see the force of Kant’s point, ask yourself this question: how can you know that your experience of reality is in any way “like” reality itself? Normally we answer this question by considering the two things separately. I can tell if my daughter’s portrait of her teacher looks like her teacher by placing the portrait alongside the person and comparing the two. I establish verisimilitude by the degree to which the copy conforms to the original. Kant points out, however, that we can never compare our experience of reality to reality itself. All we have is the experience, and that’s all we can ever have. We only have the copies, but we never have the originals. Moreover, the copies come to us through the medium of our senses, while the originals exist independently of our means of perceiving them. So we have no basis for inferring that the two are even comparable, and when we presume that our experience corresponds to reality, we are making an unjustified leap. We have absolutely no way to know this.

It is essential, at this point, to recognize that Kant is not diminishing the importance ofexperience or of the phenomenal world. That world is very important, if only because it is all we have access to. It constitutes the entirety of our human experience and is, consequently, of vital significance for us. It is entirely rational for us to believe in this phenomenal world, and to use science and reason to discover its operating principles. A recognized scientist and mathematician, Kant did not degrade the value of science. But he believed science should be understood as applying to the world of phenomena rather than to the noumenal or “other” world.

Many critics have also understood Kant to be denying the existence of external reality. This is emphatically not the case. Kant is not a skeptic in that sense. Other philosophers, such as Johann Fichte, went down that road, but Kant did not. For Kant, the noumenon obviously exists because it gives rise to the phenomena we experience. In other words, our experience is an experience of something. Moreover, Kant contends that there are certain facts about the world—such as morality and free will—that cannot be understood without postulating a noumenal realm. Perhaps the best way to understand this is to see Kant as positing two kinds of reality: the reality that we experience and reality itself. The important thing is not to establish which is more real, but to recognize that human reason operates only in the phenomenal domain of experience. We can know that the noumenal realm exists, but beyond that we can know nothing about it. Human reason can never grasp reality itself.

So powerful is Kant’s argument here that his critics have been able to answer him only with what may be termed the derision of common sense. When I challenged Daniel Dennett in a Wall Street Journal article to debunk Kant’s argument, he posted an angry response on his Web site in which he said that several people had adequately refuted Kant. But he didn’t provide any refutations, and he didn’t name any names.” Basically, Dennett was relying on the argumentum ad ignorantium, the argument that relies on the ignorance of the audience. He was hoping that his admirers would take it on faith that such refutations exist somewhere in the literature. In fact, there are no such refutations.