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.