This is a complicated book, but it's simplified by its division into multiple parts. The main point of the essays is to explain a metaphysical interpretation of reality, starting from what can be observed.
Part I: "Transcendental Doctrine of Elements"
Kant analyzes the elemental machinery of human experience.
He begins with an essay called "Transcendental Aesthetic," in which he analyzes the human's ability to determine information about the world by transcendental observation—the a priori perception of the world through external senses. By transcendental, Kant means that a human's knowledge about the world seems to extend beyond his own immediate surroundings. In other words, when human's experience reality, they accept it as reality and try to recreate it in their heads. This makes a division between the physical object (matter) and what that experience represents about reality (form).
Kant then discusses Space and Time, both of which constitute a person's literal location in reality. He treats these dimensions from a metaphysical approach, and from a transcendental approach. The metaphysical treatment shows how our knowledge of the world can be elaborated upon to reach new trustworthy conclusions, not from direct observation, but due to the metaphysical structure of existence. (Conclusions can be safely drawn from evidence). The transcendental approach shows that the process by which humans come to understand metaphysical principles must be a priori. This means that humans are built to make sense of unseen patterns and principles, intuitively. Importantly, Kant notes that Time is not a logical concept, but a transcendental one, since it does not conform to binary thinking.
Then, in "Transcendental Logic," Kant discussed rationality and the universe. In the First Division, Kant argues for a transcendental understanding of logic in a similar manner to the prior essay, discussing the metaphysical aspect of his idea and then the transcendental aspect of his idea (which he calls deductions). He also comments on the schematic structure of both nature and knowledge, and he adds a Refutation of Idealism. Apparently, he believed that people would misunderstand his philosophy as Platonic idealism.
He then turns to a Second Division, one he calls "Transcendental Dialectic." He starts by explaining the fallacies of 'pure' reasoning, arguing that logic must be tethered sincerely to one's real experience. This leads to a discussion of the soul in four parts: The Soul is Substance, The Soul is Simple, The Soul is a Person, and The Soul is Separated From The Experienced World. In those, he argues that the human "soul" must correspond to a phenomenon with substance, not purely metaphysical alone, and also that the soul is more or less simple to deduce, and that it is personal and different than reality by way of separation. Using this dialectic understanding of the soul (the soul communicates with an outside world), Kant explains how logical paradoxes can occur (he calls them antinomies, meaning "opposite laws"). Since reason can only apply to issues of experience, there is no truth to the ideas men hold about the universe, its origin, its 'meaning,' our perceived 'fate or freedom'—all those arguments regard subject matters that transcend human logic.
He concludes Part I with a discussion of reason itself, using three case studies: he rejects Anselm's Ontology, he rejects the "Prime Mover" understanding of God, and he discusses the view of God as a watch maker, since the manner through which "God" acts through reality is that it seems already programmed by the time we get here, and he doesn't have to do much to keep it going, because he designed it to be logical and circular.
Part II: "Transcendental Doctrine of Method"
Kant concludes his essays with a thorough discussion of "pure reason," breaking his thoughts into four distinct methods: "The Discipline of Pure Reason," "The Canon of Pure Reason," "The Architectonic of Pure Reason," and finally, "The History of Pure Reason."
"The Discipline of Pure Reason" takes a lot of discipline just to read. It's an attempt to exhaustively argue through the relationship between mathematical theory and philosophy. This represents Kant's analytical philosophy, whereas in the first portion of the book, his thoughts are continental and abstract.
For purposes of Canon, Kant observes that above all else, one must be able to interact rationally with the world, meaning that thoughts about God and theology must be reserved for moralistic discussions, not discussions of practical philosophy. The "Architectonic" is the background compatibility of different types of information in the grand scheme of existence. Here's the best representation of this structure that Kant can offer: The world is ontological, then physically rational, then cosmologically rational, and then finally, theological. Any discussion of "God" that doesn't represent all four realms is one that Kant disagrees with fundamentally.
In "The History," Kant explains that our ability to see the world clearly has evolved from man's fascination with God, and that strangely, so little of our philosophy has been guided by rational criticism.
