What are phenomena and noumena?

Immanuel Kant, answered from the texts and cited to the page.

Immanuel Kant answers

The distinction is this: everything that presents itself to us in experience — every object we perceive, measure, or reason about scientifically — is a phenomenon (appearance), structured by the a priori forms of our sensibility, space and time, and by the pure concepts of the understanding such as substance and cause. Our knowledge of the experienced world is so profoundly shaped by the needs of our cognitive faculties that we cannot reasonably take it as knowledge of things as they are in themselves, but only of things as they must and do appear to us.1

The noumenon, by contrast, names what lies beyond that boundary. In its negative sense — which is the only sense I can honestly defend — it is simply the concept of a thing considered apart from all relation to our mode of intuition: a limit-concept, a reminder that what we cognize is appearance, not the underlying substrate.2 The understanding, by assuming appearances at all, already grants that something underlies them; we confess thereby that appearances are based upon a thing in itself, though we know not this thing in its internal constitution, but only the way in which our senses are affected by this unknown something.3

To deny that there is any such substrate would collapse the distinction between appearance and illusion — a confusion I am at pains to avoid. The noumenon in the positive sense would be an object given to a non-sensuous, intellectual intuition — cognized directly by the understanding alone, without the mediation of space, time, or sensation.

But such an intuition is no part of our cognitive faculty.4 The categories — causality, substance, reality — possess significance only in relation to the unity of intuitions in space and time; where that unity is absent, as it is with noumena, the whole use and indeed the whole meaning of the categories is entirely lost.5 The possibility of a thing can never be proved from the mere non-self-contradictoriness of its concept; it requires an intuition corresponding to the concept, and for noumena we have none.6

There is, I acknowledge, something seductive in the pure concepts of the understanding that tempts us toward a transcendent use — a use that transcends all possible experience. The concept of cause implies a rule according to which one state follows another necessarily, and experience can only show us that one state commonly follows another, affording neither strict universality nor necessity; so the category seems to reach further than experience can take it.7

But this seduction is precisely the danger the critical philosophy was written to diagnose. Our pure concepts of the understanding, as well as our pure intuitions, extend to nothing but objects of possible experience — and as soon as we leave that sphere, these concepts retain no meaning whatever.8 Understanding and sensibility can determine objects only in conjunction.

Separate them, and we have either intuitions without concepts, or concepts without intuitions — representations that cannot be applied to any determinate object.9 The noumenon is therefore not a second world standing alongside the phenomenal one; it is the boundary-marker of the first, the concept that prevents us from mistaking the conditions of our knowledge for the unconditioned nature of things.

Sources

  1. our knowledge of the experienced world is so profoundly shaped by the needs of our cognitive faculties, we cannot reasonably take it as knowledge of things as they are in themselves, but only of things as they must and do appear to us.
    Religion within the Boundaries of Mere Reason and Other Writings, p. 10
  2. noumenon must be understood by us as such in a negative sense.
    Critique of Pure Reason, p. 62
  3. we indeed, rightly considering objects of sense as mere appearances, confess thereby that they are based upon a thing in itself, though we know not this thing in its internal constitution, but only know its appearances, viz., the way in which our senses are affected by this unknown something.
    Prolegomena to Any Future Metaphysics, p. 11
  4. an intellectual intuition, to wit, which does not, however, belong to us, of the very possibility of which we have no notion—and this is a noumenon in the positive sense.
    Critique of Pure Reason, p. 62
  5. the understanding at the same time comprehends that it cannot employ its categories for the consideration of things in themselves, because these possess significance only in relation to the unity of intuitions in space and time... Where this unity of time is not to be met with, as is the case with noumena, the whole use, indeed the whole meaning of the categories is entirely lost.
    Critique of Pure Reason, p. 62
  6. the possibility of a thing can never be proved from the fact that the conception of it is not self-contradictory, but only by means of an intuition corresponding to the conception.
    Critique of Pure Reason, p. 62
  7. The concept of cause implies a rule, according to which one state follows another necessarily; but experience can only show us, that one state of things often, or at most, commonly, follows another, and therefore affords neither strict universality, nor necessity.
    Prolegomena to Any Future Metaphysics, p. 11
  8. our pure concepts of the understanding as well as our pure intuitions extend to nothing but objects of possible experience, consequently to mere things of sense, and as soon as we leave this sphere these concepts retain no meaning whatever.
    Prolegomena to Any Future Metaphysics, p. 11
  9. Understanding and sensibility, with us, can determine objects only in conjunction. If we separate them, we have intuitions without conceptions, or conceptions without intuitions; in both cases, representations, which we cannot apply to any determinate object.
    Critique of Pure Reason, p. 62