I have yet to read any convincing evidence that we should accept Kant’s assertion that a world outside our senses exists.

I think Occam’s Razor tells us it is simpler to assume there is just one world – the phenomenal world - or to put it another way, that the noumenal world and the phenomenal world are one in the same.

Why are people apparently impressed with Kant’s assertion that a noumenal world exists?

I mean, sure, the noumenal world might be an interesting idea. But interesting in no way equates to truth. Isn’t that obvious? Or is there something I'm missing within Kant's writings where he offers convincing proof for his claim that a noumenal world exists? Isn't that, by definition, impossible? And if it's impossible to prove, why does it appear to be treated by some like a respectable idea?

Out of respect for the "one question per question rule", please consider the above musings as serving my one ultimate question here:

What is Kant's Proof for the Existence of the Noumenal World?

  • 1
    Kant makes no such assertion. To the contrary, his critique is of the pretenses of metaphysics to discern what the world behind the appearances is like. "Existence", "world" are categories of experience, and applying them beyond any possible experience, to the thing in itself, is illegitimate, according to Kant. As for the noumena, they are ideal entities posited by our reason to unify our experience, not unlike ideal limits in mathematics, see Palmquist, Two Perspectives on the Object of Knowledge.
    – Conifold
    May 17, 2019 at 4:41
  • 1
    As Conifold indicates the definition of Kant's noumenon is that it transcends existence. If it did not then his noumenon would have to have a noumenon. Existence cannot originate with an existent, and attributes cannot belong to an attribute. Perhaps one proof would be the 'problem of attributes', which cannot be solved without positing some sort of noumenon. . . . .
    – user20253
    May 17, 2019 at 10:01
  • "Achm’s Razor" ? Maybe Occam's razor. May 17, 2019 at 11:01
  • @PeterJ - Thanks for the reply. I still see Kant's claims that "things in themselves" and a noumenal world exist... as assertions without proof. As for your point that "existence cannot originate with an existent", perhaps you could clarify further, but it sounds like an assertion that won't stand up to the many seemingly illogical phenomena we find in quantum physics. "attributes cannot belong to an attribute" may be logically sound but how does that prove Kant's assertions? Positing "some sort of noumenon" in order to solve the 'problem of attributes' has the whiff of the supernatural.
    – Waterman
    May 17, 2019 at 12:00
  • I think (1) you're not grasping what phenomenal world and noumenal world mean for kant and confusing the former with an empiricist picture. (2) there's an important distinction between: (a) existing things and (b) provably existing things. The former is probably pretty large; the latter depending on the criteria you use can be empty. (3) n.b. none of this means you will come away agreeing with Kant ...
    – virmaior
    May 17, 2019 at 12:05

1 Answer 1


I suggest caution in drawing out Kant's connection with the concept of the noumenon.

    1. Kant suggests that the concept of noumena can be defended on two grounds; (a) First, its logical possibility: A254-B310. "The concept of noumenon - that is, of a thing which is not to be thought as object of the senses but as a thing in itself, solely through a pure understanding, is not in any way contradictory. For we cannot assert of sensibility that it is the sole possible kind of intuition." (b) Secondly, the need to limit sensible experience: A254-B310. "Further, the concept of a noumenon is necessary to prevent sensible intuition from being extended to things in themselves, and thus to limit the objective validity of sensible know ledge."
    2. A244-B302: ". . .to substitute the logical possibility of the concept (namely that the concept does not contradict itself) for the transcendental possibility of things (namely that an object corresponds to the concept) can deceive and leave satisfied only the simple-minded." That takes care of defense (a) above. Note also that in 1 (b) Kant makes a distinction between our sensibility, which is only one kind of intuition, and intuition, and claims we cannot say our sensibility is the only kind of intuition. Implicit is the idea that to be is to turn up in some intuition: being is appearing, and the thing in itself merely serves as a guard against limiting things to the characters they present in our intuition.
    3. In B307 the noumenon is referred to as "the entirely indeterminate concepts of an intelligible entity" which often is made into a "determinate concept of an entity that allows of being known in a certain (purely intelligible) manner by means of the understand ing." This making is declared illegitimate. The point is that a noumenon is now a very indeterminate thing.
    4. In B307 Kant distinguishes between noumenon in a positive sense (object present to nonsensous intuition, which is the first edition meaning, sometimes) and in a negative sense, namely as a limit to our sensibility, and concludes at B308: "That therefore which we entitle noumenon must be understood as being such only in a negative sense."
    5. Even of this negative sense of noumenon Kant says at A255-B310: "We are unable to comprehend how such noumena can be possible, and the domain that lies out beyond the sphere of appearances is empty. . . . The concept of a noumenon is thus merely a limiting concept, the function of which is to curb the pretensions of sensibility, and it is therefore only of negative employment. At the same time it is no arbitrary invention; it is bound up with the limitation of sensibility, though it cannot affirm anything positive beyond the field of sensibility." (ital. added)

The interesting thing in this passage is the declaration that the domain beyond the sphere of appearances is not full of things in themselves, but is empty.

    1. Finally, A256-B311 (and note that this is a text common to both editions, and thus even in the first edition, contradicts A249 which I cited earlier) : "The division of objects into phenomena and noumena and the world into a world of the senses and a world of the understanding, is therefore quite inadmissible in the positive sense. . . . Nonetheless, if the concept of a noumenon be taken in a merely problematic sense, it is not only admissible, but as setting limits to sensibility is likewise indispensable. But in that case a noumenon is not for our understanding a special (kind) of object, namely an intelligible object; the (sort of) understanding to which it might belong is itself a problem." (ital. added)

In his own words the noumenon is not an object or a kind of object, nor is its nature clear; it is a problem.

I submit that the end of this line of thought would be to drop the phrase "thing in itself" completely and to replace it with a term which does not have so many confusing consequences. Further, I submit that Kant was aware of this, that he did come to a more functional rather than substantial interpretation of the thing in itself, and that he struggled to give expression to it. Running through all of the concepts of the noumenon, even the substantial ones in the first edition, is the fundamental theme of a function within experience, a dimensional aspect of our awareness of phenomenal presence. It is necessary to limit the sensibility, to "curb the pretensions" of the category-structured human intuition. Aside from this function Kant in the end could not assign any definite meaning to the noumenon, and finally he admitted it. It just cannot be conceived in any sense as an object. Nor, I think, can it ultimately be considered as reality in any way as opposed to appearance, if this reality remains utterly inaccessible to and ineffable within experience. Hence, a purely experiential function must be found for it. Kant is an empiricist and always maintains that only that which relates to experience is meaningful, only that which turns up in the intuition is real. (Richard F. Grabau, 'Kant's Concept of the Thing in Itself: An Interpretation', The Review of Metaphysics, Vol. 16, No. 4 (Jun., 1963), pp. 770-779: 774-6.)


Richard F. Grabau, 'Kant's Concept of the Thing in Itself: An Interpretation', The Review of Metaphysics, Vol. 16, No. 4 (Jun., 1963), pp. 770-779.

Kant, Critique of Pure Reason, N. Kemp Smith tr. The later translation by Paul Guyer and Allen W. Wood, Cambridge: CUP, 1998, is preferable but I have had to keep to Grabau's text and the modern Cambridge translation is readily available for cross-checking.

  • Thank you for the well researched answer. It seems (particularly by points 4,5, and 6) that Kant did not believe in the existence of an actual noumenal world, but instead used the concept to underline the point that our senses have limits, and that we can only experience the world through these sensory limitations, thus our experience is not "the complete picture". This theoretical "complete picture" outside our senses is what Kant means by speaking of a noumenal world.
    – Waterman
    May 18, 2019 at 13:26
  • If so, then allow me to solidify with an example. You and I see a red apple. Kant would say that it's only red to us because that is how our eyes and brain interpret it, thus in our phenomenal worlds (internal cognition) it is red, but a colour blind person would say it is brown. If these sensory imperfections are what Kant intends to point out by distinguishing between the phenomenal and noumenal worlds then I agree with him. That sort of "incompleteness" in our pictures of the world seems obvious, and is borne out by science. Is this what Kant intends by his phenomenal and noumenal "worlds"?
    – Waterman
    May 18, 2019 at 13:37
  • @Waterman. I'm not ignoring or otherwise avoiding you comments, just thinking about them. I'll get back to you ASAP. Best - Geoffrey
    – Geoffrey Thomas
    May 19, 2019 at 12:47
  • No worries. Fast food isn't good for us. Generally speaking I would say the same about fast thought.
    – Waterman
    May 21, 2019 at 23:57
  • Marvelllous comparison - I'm still smiling ! Thanks. Best - Geoffrey
    – Geoffrey Thomas
    May 22, 2019 at 8:59

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