Man must be disciplined, for he is by nature raw and wild.

This quote embodies the spirit of Kant’s Critique of Pure Reason and its concern with bringing order and clarity to the debate on metaphysics.

To know what questions may reasonably be asked is already a great and necessary proof of sagacity and insight. For if a question is absurd in itself and calls for unnecessary answers, it not only brings disgrace to the person raising it, but may prompt an incautious listener to give absurd answers, thus presenting, as the ancients said, the laughable spectacle of one person milking a he-goat, and another holding the sieve underneath.

Kant’s desire for a firm and dependable basis for his observations is here evident. Critique of Pure Reason is, above all else, concerned with how things are known, what knowledge is, and how it is to be pursued. Posing metaphysical questions without understanding these truths was, for Kant, likely to lead to confusion, both for the questioner and for the respondent.

That all our knowledge begins with experience there can be no doubt. For how should the faculty of knowledge be called into activity, if not by objects which affect our senses and which, on the one hand, produce representations by themselves or on the other, rouse the activity of our understanding to compare, connect, or separate them and thus to convert the raw material of our sensible impressions into knowledge of objects, which we call experience? But though all our knowledge begins with experience, is does not follow that it all arises from experience. For it is quite possible that even our empirical knowledge is a compound of that which we perceive through impressions, and of that which our own faculty of knowledge (incited by sense impressions) supplies.

Here, Kant illustrates the first of the two key distinctions discussed in Critique of Pure Reason between a priori knowledge—that which is known without the need for empirical verification—and a posteriori knowledge—that which is known only after such verification. Kant takes the middle ground between the rationalist position, upheld by philosophers (such as Descartes) who saw a priori reason as the key to understanding all things, and the Empiricist position, upheld by philosophers (such as David Hume) who saw empirical experiment as essential for knowing anything. Kant argues that while knowledge is always a product of “experience,” experience is a combination of what we experience with the senses and how this data is interpreted and filtered by our a priori reason.

If I say: “All bodies are extended,” then this is an analytic judgment. For I do not need to go outside the concept that I combine with the word “body” in order to find that extension is connected with it, but rather I need only to analyze that concept, i.e., become conscious of the manifold that I always think in it, in order to encounter this predicate therein; it is therefore an analytic judgment. On the contrary, if I say: “All bodies are heavy,” then the predicate is something entirely different from that which I think in the mere concept of a body in general. The addition of such a predicate thus yields a synthetic judgment.

Here is illustrated the second of the key distinctions fundamental to Critique of Pure Reason: that between analytic and synthetic judgements. Statements which are self-evident, which actually contain their own truth within their terms, are analytic. Statements which do not contain their own truths within themselves and that therefore require some further verification are synthetic.

I have no knowledge of myself as I am but merely as I appear to myself.

Here Kant introduces his distinction between Phenomena and Noumena—the former meaning our sensory experience of the world around us and ourselves, and the latter meaning the reality of things as they are independent of our observations.

The order and regularity in the appearances, which we entitle nature we ourselves introduce.

This statement illustrates Kant’s belief that human beings do not, and cannot, passively observe the noumena in nature as they are. Rather, our observations create what we see in the world as phenomena.