In his book The World as Will and Idea, philosopher Arthur Schopenhauer writes, “Kant’s teaching produces a fundamental change in every mind that has grasped it. The change is so great that it may be regarded as an intellectual rebirth…. In consequence of this, the mind undergoes a fundamental undeceiving, and thereafter looks at things in another light.” The greatness of Kant is that he takes our most fundamental assumptions and turns them into questions. We think we are on the ground floor of awareness, but Kant shows us a whole different level beneath it that we can examine.

Before Kant, most people simply assumed that our reason and our senses give us access to external reality—the world out there—and that there is only one limit to what human beings can know That limit is reality itself. In this view, still widely held by many in our society, human beings can use the tools of reason and science to continually find out more and more until eventually there is nothing else to discover. The Enlightenment fallacy holds that human reason and science can, in principle, gain access to and eventually comprehend the whole of reality.

In his Critique of Pure Reason, Kant shows that these assumptions are false. In fact, he argues, there is a much greater limit to what human beings can know. In other words, human reason raises questions that—such is the nature of our reason—it is incapable of answering. And it is of the highest importance that we turn reason on itself and discoverwhat those limits are. It is foolishly dogmatic to go around asserting claims based on reason without investing what kinds of claims reason is capable of adjudicating. Reason, in order to be reasonable, must investigate its own parameters.

Kant begins with a simple premise: all human knowledge is based on experience. We gain access to reality through our five senses. This sensory input is then processed through our brains and central nervous systems. Think about it: every thought, even the wildest products of our imagination, are exclusively based on things that we have seen, heard, touched, smelled, or tasted. If we imagine and draw creatures from outer space, we can give them four eyes and ten legs, but ultimately we have no way to conceive or portray them except in terms of our human experience. It is an empirical fact that our five senses are our only lenses for perceiving reality.

Now Kant asks a startling question: how do we know that our human perception of reality corresponds to reality itself? Most philosophers before Kant had simply taken for granted that it does, and this belief persists today. So powerful is this “common sense” that many people become impatient, even indignant, when Kant’s question is put to them. They act as if the question is a kind of skeptical ploy, like asking people to prove that they really exist. But Kant was no skeptic: he saw himself as providing a refutation of skepticism. He knew, however, that to answer skepticism one has to take the skeptical argument seriously. The way to overcome skepticism is by doing justice to the truth embodied in it. Kant’s goal was to erect a dependable edifice for knowledge on the foundation of extreme skepticism.

Kant’s question about the reliability of human perception has been the central preoccupation of Western philosophy since Descartes. How do we know what we claim to know? Locke had famously pointed out that material objects seem to have two kinds of properties, what he called primary properties and secondary properties. Primary properties are in the thing itself, whereas secondary properties are in us. So when we perceive an apple, for example, its mass and shape are part of the apple itself. But Locke ingeniously pointed out that the redness of the apple, its aroma, and its taste are not in the apple. They are in the person who sees and smells and bites into the apple. What this means is that our knowledge of external reality comes to us from two sources: the external object and our internal apparatus of perception. Reality does not come directly to us but is “filtered” through a lens that we ourselves provide.