Kant published the Critique of Pure Reason in 1781. It is very long and almost unreadable due to its dry prose and complex terminology. Kant tried to ease his readers’ confusion by publishing the Prolegomena to Any Future Metaphysics two years later. While it is hardly a page-turner, the Prolegomena is much briefer than the Critique and much more accessible in style, making it a valuable entry point to Kant’s metaphysics and epistemology.

Kant’s primary aim is to determine the limits and scope of pure reason. That is, he wants to know what reason alone can determine without the help of the senses or any other faculties. Metaphysicians make grand claims about the nature of reality based on pure reason alone, but these claims often conflict with one another. Furthermore, Kant is prompted by Hume’s skepticism to doubt the very possibility of metaphysics.

Kant draws two important distinctions: between a priori and a posteriori knowledge and between analytic and synthetic judgments. A posteriori knowledge is the particular knowledge we gain from experience, and a priori knowledge is the necessary and universal knowledge we have independent of experience, such as our knowledge of mathematics. In an analytic judgment, the concept in the predicate is contained in the concept in the subject, as, for instance, in the judgment, “a bachelor is an unmarried man.” (In this context, predicate refers to whatever is being said about the subject of the sentence—for instance, “is an unmarried man.”) In a synthetic judgment, the predicate concept contains information not contained in the subject concept, and so a synthetic judgment is informative rather than just definitional. Typically, we associate a posteriori knowledge with synthetic judgments and a priori knowledge with analytic judgments. For instance, the judgment “all swans are white” is synthetic because whiteness is not a part of the concept of “swan” (a black swan would still be a swan even though it isn’t white), but it is also a posteriori because we can only find out if all swans are white from experience.

Kant argues that mathematics and the principles of science contain synthetic a priori knowledge. For example, “7 + 5 = 12” is a priori because it is a necessary and universal truth we know independent of experience, and it is synthetic because the concept of “12” is not contained in the concept of “7 + 5.” Kant argues that the same is true for scientific principles such as, “for every action there is an equal an opposite reaction”: because it is universally applicable, it must be a priori knowledge, since a posteriori knowledge only tells us about particular experiences.

The fact that we are capable of synthetic a priori knowledge suggests that pure reason is capable of knowing important truths. However, Kant does not follow rationalist metaphysics in asserting that pure reason has the power to grasp the mysteries of the universe. Instead, he suggests that much of what we consider to be reality is shaped by the perceiving mind. The mind, according to Kant, does not passively receive information provided by the senses. Rather, it actively shapes and makes sense of that information. If all the events in our experience take place in time, that is because our mind arranges sensory experience in a temporal progression, and if we perceive that some events cause other events, that is because our mind makes sense of events in terms of cause and effect. Kant’s argument has a certain parallel to the fact that a person wearing blue-tinted sunglasses sees everything in a bluish light: according to Kant, the mind wears unremovable time-tinted and causation-tinted sunglasses, so that all our experience necessarily takes place in time and obeys the laws of causation.

Time and space, Kant argues, are pure intuitions of our faculty of sensibility, and concepts of physics such as causation and inertia are pure intuitions of our faculty of understanding. Sensory experience only makes sense because our faculty of sensibility processes it, organizing it according to our intuitions of time and space. These intuitions are the source of mathematics: our number sense comes from our intuition of successive moments in time, and geometry comes from our intuition of space. Events that take place in space and time would still be a meaningless jumble if it were not for our faculty of understanding, which organizes experience according to the concepts, like causation, which form the principles of natural science.