In Immanuel Kant's Critique of Pure Reason is a reconstruction of the then predominant school of skeptical empiricism, which typified almost all of seventeenth-century European philosophy from the works of Renee Descartes to George Berkley to David Hume. Most thinkers of Kant’s day believed that ideas in the mind took the “shape” of the objects in the external world, which were revealed to them via the sense perceptions. The mind, these philosophers argued, was something like a repository for ideas, and that upon introduction to external objects, the mind modified itself in order to best understand them. Critique of Pure Reason inverted this perspective. Instead of causing a fundamental modification of the mind, Kant argued that the external objects in the world rather conform to the mind; the ways in which human beings are able to perceive their environment, he maintained, is a direct consequence of the ability of the mind to act on this environment and convert it into something meaningful to the observer.

Kant further argued that, in order for the mind to accomplish this manipulation of the external world, it must have possessed certain categories of understanding that existed outside of sense experience. These categories of thought, he said, existed a priori, or prior to observation, of the object in question. Kant argued that “space” and “time” were the two primary a priori categories that dictated the way the mind operates. These criteria must be presupposed to exist in order for an individual to be capable of having sensibility of the external world at all. Kant maintained that this was because, though space and time are not “objects” in the literal sense, they provide the internal logic by which the human mind operates in order to imbue objects in the external world with any degree of coherency, organization, and meaning. It is impossible to conceive of, say, a book without first recognizing that as a body it both extends outwards (space) and acts and reacts to other extended objects in space temporally (time). Thus, while space and time are not necessarily contained within the formal definition that one would associate with book, they are nevertheless required in order to make this definition meaningful.

In this way, Kant drew a distinction between what he called the “phenomenal world,” or the world that our mind creates for us, and the “noumenal world,” or the world as it really is, apart from our experience of it. Kant’s enduring contribution to metaphysical philosophy comes from his denial of pure knowledge. Human beings are incapable of understanding the essence of a thing outside of the way their minds modify it within the boundaries set by a priori categories. Thus, the essences of things, what Kant called “things-in-themselves” (dinge an sich) in German, cannot be arrived at through human reason alone. Instead, the “ideas” of reason, which Kant classified as God, the world, and the self, help unify (but not absolutely define) knowledge, and can point to possibilities in the noumenal world.

Critique of Pure Reason is a notoriously long and convoluted book. Kant wrote it for a highly specialized academic audience (a sixteenth-century academic audience, no less) in prose that is almost impossible to follow. His colleagues at the University of Konigsberg suggested to him that he write a condensed version of the book so that it could be made available to more of a mainstream audience. If you are really pressed for time, this 70-ish page book is a good summation of Kan’t overarching argument. It is called Prolegomena to any Future Metaphysics, and is widely available online.