A precise and concise summary of this piece of philosophical wisdom.
What is meant by this title? Critique is not precisely a criticism, but a critical analysis; Kant is not attacking "pure reason," except, at the end, to show its limitations; rather he hopes to show its possibility, and to exalt it above the impure knowledge which comes to us through the distorting channels of sense. For "pure" reason is to mean knowledge that does not come through our senses, but is independent of all sense experience; knowledge belonging to us by the inherent nature and structure of the mind.
At the very outset, then, Kant flings down a challenge to Locke and the English school: knowledge is not all derived from the senses. Hume thought he had shown that there is no soul, and no science; that our minds are but our ideas in procession and association; and our certainties but probabilities in perpetual danger of violation. These false conclusions, says Kant, are the result of false premises: you assume that all knowledge comes from "separate and distinct" sensations; naturally these cannot give you necessity, or invariable sequences of which you may be forever certain; and naturally you must not expect to "see" your soul, even with the eyes of the internal sense. Let us grant that absolute certainty of knowledge is impossible if all knowledge comes from sensation, from an independent external world which owes us no promise of regularity of behavior. But what if we have knowledge that is independent of sense-experience, knowledge whose truth is certain to us even before experience - a priori?Then absolute truth, and absolute science, would become possible, would it not? Is there such absolute knowledge?
This is the problem of the first Critique.
"My question is, what we can hope to achieve with reason, when all the material and assistance of experience are taken away."
The Critique becomes a detailed biology of thought, an examination of the origin and evolution of concepts, an analysis of the inherited structure of the mind. This, as Kant believes, is the entire problem of metaphysics.
"In this book I have chiefly aimed at completeness; and I venture to maintain that there ought not to be one single metaphysical problem that has not been solved here, or to the solution of which the key at least has not here been supplied."
Exegi monumentum aere perennius! With such egotism nature spurs us on to creation.
The Critique comes to the point at once.
"Experience is by no means the only field to which our understanding can be confined. Experience tells us what is, but not that it must be necessarily what it is and not otherwise. It therefore never gives us any really general truths; and our reason, which is particularly anxious for that class of knowledge, is roused by it rather than satisfied. General truths, which at the same time bear the character of an inward necessity, must be independent of experience - clear and certain in themselves."
That is to say, they must be true no matter what our later experience may be; true even before experience; true a priori.
"How far we can advance independently of all experience, in a priori knowledge, is shown by the brilliant example of mathematics."
Mathematical knowledge is necessary and certain, we cannot conceive of future experience violating it. We may believe that the sun will "rise" in the west tomorrow, or that some day in some conceivable asbestos world, fire will not burn stick; but we cannot for the life of us believe that two times two will ever make anything else than four. Such truths are true before experience; they do not depend on experience past, present, or to come. Therefore they are absolute and necessary truths; it is inconceivable that they should ever become untrue. But whence do we get this character of absoluteness and necessity? Not from experience; for experience gives us nothing but separate sensations and events, which may alter their sequence in the future. These truths derive their necessary character from the inherent structure of our minds, from the natural and inevitable manner in which our minds must operate. For the mind of man (and here at last is the great thesis of Kant) is not passive wax upon which experience and sensation write their absolute and yet whimsical will; nor is it a mere abstract name for the series or group of mental states; it is an active organ which moulds and coordinates sensations into ideas, an organ which transforms the chaotic multiplicity of experience into the ordered unity of thought. [Will Durant, The Story of Philosophy]