Summary: Critique of Pure Reason

The Critique of Pure Reason is a difficult work—and its difficulty arises mainly from the fact that the Preface is filled with allusions to the content of the book (from the Introduction to the final chapter) that remain incomprehensible until one has read the work in its entirety. These numerous obscurities often discourage readers from proceeding further.

The simplest approach is to begin with the Introduction, which contains essential definitions and distinctions. One can return to the Preface once these have been grasped, at which point its allusions will become clearer.

This work seeks to answer the question: What can we know? This is a fundamental question in epistemology, examined in particular by Descartes, Locke, and Hume. Do our ideas correspond to something real, or are they mere fabrications? Is the external world truly as we perceive and conceive it to be? Are our theories about the world correct?

To understand a philosopher, it is often helpful to identify the thinker against whom they are primarily arguing.

By Kant's own account, his primary opponent is Hume and, more broadly, empiricism.

For empiricism, the mind is a kind of tabula rasa (a term used by Locke)—originally containing no ideas, a blank slate. Experience is the source of all our ideas and knowledge. It is by seeing the colour red, for example, that the idea of 'red' enters our mind; and it is by experiencing the feeling of anger within ourselves that the idea of 'anger' takes shape.

Ideas are thus formed through experience—either external (that of the world around us) or internal (when the mind registers certain things within us, as in the case of anger).

Empiricists therefore reject the theory of innate ideas, which holds that we are born with certain pre-formed ideas, such as the idea of God.

Kant takes issue with the empiricist doctrine. He challenges the notion that the mind—or rather, to use his own term, the understanding—is a neutral medium in which ideas simply come to be formed.

Kant's insight is that the understanding has a particular form. It does not simply receive ideas from external things without altering them, as it would if it were a neutral tabula rasa. For something to become an object of knowledge and take shape as an idea, it must be modified so as to conform to the form of the understanding.

A useful analogy is that of a glass: for a liquid to enter the glass, it must take on its shape, whatever that may be. In the same way, for an idea to form within our understanding, it must adapt to the form of our understanding.

What is this form? It consists of a set of a priori concepts, or categories—fundamental concepts upon which the very functioning of our understanding depends. Here are a few examples (the full list will be given later): quality, quantity, causality, and so on.

For an object to be constituted as an object within our intellect, it must have a certain quantity. And indeed, all our ideas of objects correspond to something singular or multiple—in short, something possessing a certain quantity.

We can intuitively see that the idea of causality is more fundamental than, say, the idea of metal. The former is an a priori concept of the understanding; the latter is an empirical concept. One could imagine being deprived of the idea of metal, but the idea of causality is essential to our intellect.

What does the term a priori mean? A priori denotes that which precedes experience and is independent of it—as opposed to a posteriori, which refers to that which is derived from experience. A posteriori and empirical are synonymous, just as pure and a priori are synonymous (a pure concept is one that is entirely a priori).

We can now understand why these fundamental concepts of the understanding are a priori concepts. Since they constitute the very form of the understanding, they are not formed in us through experience. On the contrary, it is these concepts that enable us to think about any object of experience—and even to constitute objects as objects of experience.

¹ Our translation. The references for the quotations are available in the book Kant: A Close Reading