In the broadest terms, the Prolegomena to any Future Metaphysics That Will Be Able To Present Itself as a Science tries to answer what Kant says is the most important question facing philosophy: "what can we know?" For Kant, in other words, the goal of philosophy is to understand what knowledge is. Philosophy is a kind of meta-knowledge—knowledge about knowledge. The Prolegomena, which condenses and simplifies arguments presented in his magnum opus, The Critique of Pure Reason, attempts to draw the boundaries of legitimate knowledge, and thus establish philosophy, which he calls metaphysics, on a solid basis.
According to Kant, the problem that has plagued metaphysics from its very inception is that philosophers have used it without first discerning what it could and could not legitimately know; consequently, they have attempted to give answers that metaphysics cannot possibly give. The “dogmatists,” as Kant calls the rationalist philosophy of Gottfried Leibniz (1646-1716) and his student Christian Wolff (1679-1754), believed that reason, or abstract thought, could attain direct and irrefutable proof about the world, about God, and about human beings—e.g, that everything in existence has a cause; that God exists, that he is good, and that he is rational; and that we have immortal souls.
The dogmatists were, in their turn, countered by the skeptical empiricism of John Locke (1632-1704) and, most importantly, David Hume (1711-1776), who argued that we could only know with certainty those things of which we had direct sensory experience. Kant considered this a valuable discovery, but he also recognizes that it has driven philosophy into a dead end, since, according to Hume, we can never know for certain that one thing has caused another, or that the sun will rise tomorrow. Kant believed that this latter view was directly contradicted by the enormous strides in mathematics and physics over the previous century, which offered universally-accepted proof that, for example, any two objects combined with any other two will make four. The question, then, is how? Given the crucial role that sensory experience plays in the way we come to know things, how can we know things of which we have no experience? If metaphysics can answer that question, then metaphysics can become a proper science.
Kant’s answer is that, while most of our knowledge does, in fact, originate for our senses, our senses seem to offer us very little in the way of universal knowledge. We can have no knowledge of things as they are beyond our perception—a major concession. The matter becomes a very different one, however, when we start to ask why, and by what right, we perceive things the way we do. Kant concedes that our senses can’t offer us universal certainty, but it can furnish us with knowledge about the way our mind pre-forms our perceptions. This knowledge can, in turn, offer us objective knowledge of the world as it is perceived by us. Kant refers to this shift in the focus of philosophy his “Copernican turn.” Just as Copernicus demonstrated that the earth revolved around the sun, and not vice versa, so too did Kant claim to have demonstrated that philosophy’s proper area of inquiry was not the world, but its point of origin, the human mind.
