Understanding Noumena in Kant’s Critique of Pure Reason

In the Analytic of Principles, Kant draws a distinction between phenomena and noumena in an attempt to understand the limits of the understanding. Kant argues that appearances are undetermined objects of empirical intuition and that these appearances, as “objects of sense,” are phenomena. When we say something like “object of appearance,” and distinguish this from a thing in itself, we are implying the existence of a “thing in itself.” This “thing in itself” is what Kant calls noumena.

Noumena are either objects abstracted away from their sensible properties or they are not sensible objects. Kant argues that things which would be thought only through the understanding and not through sensible intuition, are noumena. Kant explores if it is possible to think of something only through understanding and not at all through sensibility by exploring if the pure concepts of the understanding could be applied to noumena. He argues that “all concepts and with them all principles, however a priori they may be, are nevertheless related to empirical intuitions, i.e., to data for possible experience” (A239/B298). Because the pure concepts of the understanding or the categories are the ways in which the understanding applies itself to objects that are given to us through sensible intuition, noumena, as objects which do not involve sensible intuition, can not be thought through the a priori concepts of the understanding. Kant argues that noumena, as objects of the understanding but not of sensible intuition, would have to involve some other kind of intuition which he calls intellectual intuition in order to be thought through the categories. However, humans cannot have purely intellectual intuition and therefore cannot apply the categories to noumena and cannot cognize noumena. 

As a result, Kant asserts that noumena can only be understood in the negative sense. That is, we can only understand noumena as “a thing insofar as it is not an object of our sensible intuition” (B307). According to Kant, we can know that noumena, as a “thing in itself,” exists only as an implication of the existence of phenomena as “appearances” which are not things in themselves. In this way, noumena is only a “boundary concept” in that it speaks only to the limits of our understanding. We cannot, however, have positive knowledge of noumena which would be thinking of noumena as “an object of a non-sensible intuition” (B307). In order to have a positive understanding of noumena, we would have to be able to apply the categories to it and we would therefore have to receive noumena through some form of non-sensible intuition, namely “intellectual intuition.” Since we do not have intellectual intuition, we cannot cognize noumena and therefore cannot know anything about it or its content. While we can know that noumena must exist in the negative sense because the very idea of an “appearance” of a thing as something different from the thing in itself implies a thing in itself, ie. noumena, we cannot know anything about noumena other than that it is not cognizable. 

Those “things in themselves” which do not come to us through sensible intuition and therefore cannot be understood through the use of the categories are noumena and cannot be cognized. This is important because Kant argues that our study of metaphysics must look only to the phenomenal since we cannot know anything outside of the application of our understanding to it.