What atheists Kant refute

Reason must know its limits in order to be truly reasonable.

Religion has faced formidable foes in its history. But atheism hasn't generally been one of them – until today. A recent string of bestselling books has put believers of all stripes on the defensive. Religion, say authors such as Richard Dawkins, Daniel Dennett, and Christopher Hitchens, is an unreasonable form of blind faith, often leading to fanaticism and violence. Reason and science, they contend, are the only proper foundations for forming opinions and understanding the universe. Those who believe in God, they insist, are falling for silly superstitions.

This atheist attack is based on a fallacy – the Fallacy of the Enlightenment. It was pointed out by the great Enlightenment philosopher Immanuel Kant. Kant erected a sturdy intellectual bulwark against atheism that hasn't been breached since. His defense doesn't draw on sacred texts or any other sources of authority to which people of faith might naturally and rightfully turn when confronted with atheist arguments. Instead, it relies on the only framework that today's atheist proselytizers say is valid: reason. The Fallacy of the Enlightenment is the glib assumption that there is only one limit to what human beings can know – reality itself. This view says we can find out more and more until eventually there is nothing more to discover. It holds that human reason and science can, in principle, unmask the whole of reality.

In his 1781 "Critique of Pure Reason," Kant showed that this premise is false. In fact, he argued, there is a much greater limit to what human beings can know. Kant showed that human knowledge is constrained not merely by the unlimited magnitude of reality but also by a limited sensory apparatus of perception.

Consider a tape recorder. It captures only one mode of reality, namely sound. Thus all aspects of reality that cannot be captured in sound are beyond its reach. The same, Kant would argue, is true of human beings. The only way we apprehend empirical reality is through our five senses. But why should we believe, Kant asked, that this five-mode instrument is sufficient? What makes us think that there is no reality that lies beyond sensory perception?

Moreover, the reality we apprehend is not reality in itself. It is merely our experience or "take" on it. Kant's startling claim is that we have no basis for assuming that a material perception of reality ever resembles reality itself. I can tell if my daughter's drawing of her teacher looks like the teacher by placing the portrait alongside the person. With my eyes, I compare the copy with the original. Kant points out, however, that comparing our experience of reality to reality itself is impossible. We have representations only, never the originals. So we have no basis for presuming that the two are even comparable. When we equate experience and reality, we are making an unjustified leap.

It is essential to recognize that Kant isn't diminishing the importance of experience. It is entirely rational for us to use science and reason to discover the operating principles of the world of experience. This world, however, is not the only one there is. Kant contended that while science and reason apply to the world of sensory phenomena, of things as they are experienced by us, science and reason cannot penetrate what Kant termed the noumena – things as they are in themselves.

Some critics have understood Kant to be denying the existence of external reality or of arguing that all of reality is "in the mind." Kant emphatically rejects this. He insists that the noumenon obviously exists because it is what gives rise to phenomena. In other words, our experience is an experience of something. Perhaps the best way to understand this is to see Kant as positing two kinds of reality: the material reality that we experience and reality itself. To many, the implication of Kant's argument is that reality as a whole is, in principle, inaccessible to human perception and human reason.

So powerful is Kant's argument here that his critics have been able to answer him only with derision, as though his arguments are self-evidently fallacious. When I challenged Daniel Dennett to debunk Kant's argument, he responded on his website by saying several people had already refuted Kant. But he didn't provide any refutations and he didn't name any names. Basically, Mr. Dennett was relying on the ignorance of the audience. In fact, there are no such refutations.

Although Kant's argument seems counterintuitive – in the way that some of the greatest ideas from Copernicus to Einstein are counterintuitive — no one who understands the central doctrines of the world's leading religions should have any difficulty grasping his main point. Kant's philosophical vision is largely congruent with the teachings of many faiths that the empirical world is not the only world. Ours is a world of appearances only, in which we see things in a limited and distorted way – "through a glass, darkly," as the apostle Paul writes in I Corinthians. The spiritual reality constitutes the only permanent reality there is. Christianity teaches that while reason can point to the existence of this higher domain, it cannot on its own fully comprehend that domain.

Thus, when Christopher Hitchens and other atheists routinely dismiss religious claims on the grounds that "what can be asserted without evidence can also be dismissed without evidence," they are making what philosophers like to call a category mistake. We learn from Kant that within the domain of experience, human reason is sovereign, but it is in no way unreasonable to believe things on faith that simply cannot be adjudicated by reason.

When atheists summarily dismiss such common ideas as the immortality of the soul or the afterlife on the grounds that they have never found any empirical proofs for either, they are asking for experiential evidence in a domain that is entirely beyond the reach of the senses. In this domain, Kant argues, the absence of such evidence cannot be used as the evidence for absence.

Notice that Kant's argument is entirely secular: It does not employ any religious vocabulary, nor does it rely on any kind of faith. But in showing the limits of reason, Kant's philosophy "opens the door to faith," as the philosopher himself noted.

Kant exposes the ignorant boast of atheists that atheism operates on a higher intellectual plane than theism. He shows that reason must know its limits in order to be truly reasonable. Atheism foolishly presumes that reason is in principle capable of figuring out all that there is, while theism at least knows that there is a reality greater than, and beyond, that which our senses and our minds can ever apprehend.

• Dinesh D'Souza's new book "What's So Great About Christianity," is out this week.