This is a daunting work. It's also a necessary work, inasmuch as any understand of contemporary thought and intellectual history must encounter it. Kant has influenced nearly every major school of thought and cultural trend for the last 200 years. Below, I'll try to sketch his thought in this Critique.
This is the story of Immanuel Kant, who found philosophy a mess and sought to fix it. Specifically, he was a former Rationalist who was disconcerted by British Empiricism (specifically the skeptical philosophy of David Hume). He sought to provide a grounding for the truths of empirical science and mathematics, establish the possibility of religious faith and practice, while at the same time avoid dogmatism in metaphysical reasoning.
How did he seek to do this? By establishing a critique of reason whereby he understands the validity of all mental constructs. Kant distinguish between judgments which are a priori (prior to experience) and a posteriori (arising out of experience), and judgments which are "analytic" (trivial, tautological) and "synthetic" (where the predicate adds something that is not contained within the subject). Are synthetic a priori judgments possible? Kant answers yes, and much of this book deals with what follows from that.
First Kant deals with how we have sense experience. He claims that space and time are necessary a priori conditions for sense experience -- not physical things in the world. The content of our experience is sense-data: raw sensation that arises outside ourselves or inside ourselves and is "given" in experience. The forms in which we construct that experience are space and time. Sensations, organized within us spatially and temporally yield sense experience (perceptions).
Kant then proceeds to our abstract thought. What he terms "Understanding" has pure, a priori concepts according to logical form. He calls these "Categories." These do NOT arise as a mere empirical habit/convention -- they are prior to experience and are necessary forms that allow rational beings to experience the world intelligibly. Thus, we take the raw givens of our Understanding, which are perceptions (which we dealt with under "Transcendental Aesthetic"), and we impose the categories upon these perceptions -- we "schematize" our experience.
Perceptions, when given intelligible form according to schemata, yield intelligible concepts. We are justified in doing this because the perceptions are not things-in-themselves, but mere appearances (phenomena), and in order for these phenomena to exist in an experience that is coherent and consistent for us, they must have these forms. We are NOT justified in applying these categories to things-in-themselves (noumena).
This is where Reason eats itself. It tries to do the same thing the Understanding did, but now it does this with respect to the big metaphysical questions. It starts with concepts and attempts to unify all phenomenal experience according to concepts and yield the Ideas of Pure Reason. When it does this, it gets all confuzelled. It tries to deal with 3 Big Problems (Kant uses the term "dialectic"):
- Soul - Reason wants to insist that the thinking soul exists, that it is subject (pure substance), that it is simple, and that it is unchangeable through all its activities. These are the Paralogisms of Pure Reason. We need these ideas -- their contraries are unthinkable for us(?), but these are not demonstrable.
- The World - Reason wants to answer questions about the series of appearances that constitute the world: Is the World limited or unlimited in space and time? Is the world made up of simples or composites? Does freedom exist in the world? Is there a necessary being connected with the world? These are the Antinomies of Pure Reason. Unlike the Paralogisms, these questions admit of contradictory answers. They, too, cannot be adjudicated by pure reason.
- God - Reason wants to demonstrate the existence of God. Kant refers to this as the Ideal of Pure Reason. He claims that all arguments demonstating God's existence in fact, despite outward appearances, depend upon one method, the "ontological" proof of God's existence, which Kant disallows as transempirical.
Kant tries to tell us how to employ reason. First, stop arguing speculatively about God and other ultimate metaphysical realities! But he does urge us to apply those metaphysical ideas in practical (moral) contexts -- we are obligated to do so. In this, he anticipates the Victorians, who were somewhat skeptical on matters of faith, but stressed the necessity of continuing to act according to traditional morality. The dialectic problems deals with ideas are not verifiable speculatively. They are not constitutive of experience. Rather, they serve a regulative function, specifically in the practical realm of morality.
Kant claims that reason is architectonic: it naturally wants to assume the greatest generality. Kant says this is fine for moral thinking, but bad for speculative thinking.
Kant says that philosophy answers these questions: "What can I know? What ought I to do? What may I hope for?" The bulk of Critique of Pure Reason answers the first question. The Critique of Practical Reason, Groundwork of the Metaphysic of Morals, Metaphysic of Morals, etc., answer the second question. The third question ties the two together -- this is what Kant deals with at the end of the first Critique.
Kant sees the great transendental ideas as being God, Immortality, and Freedom. They are the starting points of theistic religion (e.g. Christianity and Judaism). These can neither be verified nor disproved by speculative reason (since speculative reason must by its nature deal with givens (Latin, data) either from sense-experience or pure intuition (as in mathematics). These ideas, however, are necessary "regulative" ideas for the guidance of practical (moral reason) and are valid in that connection. Thus, the second Critique answers the question "What ought I to do?" by recourse to the transcendal idea of Freedom. The question, "what may I hope for?", is given response through the transcendental ideas of God and immortality, for if God does not exist, nothing can grant us happiness for moral behavior and unhappiness for immoral behavior, and if we're not immortal, God won't have anyone to reward.
I probably have made errors and inaccuracies in the above, but I hope I give a flavor for his thought. Kant is sober, earnest, and disciplined. Again, he's not easy, but I think he's worth the effort.