I understand how Kant arrives at the existence of synthetic apriori truths, in opposition to Hume's relation of ideas/matters of fact distinction.

I also understand the overall epistemic distinction between noumenon and phenomenon. But I struggle with the technical aspect of how the first concept implies the second.

How does Kant make the leap from the existence of synthetic apriori to the noumenon/phenomenon distinction ?

  • To rephrase a bit: How can we form concepts and make judgments prior to experience? Because everything we know as "experience" is already "partly" formed out of sense-making intuitions and intelligible concepts. Just as it takes a lens and retina to make "images." But what then is the other part of this "partly"? What causes the sensations we form into appearances. What is an "image" really... without an interfering eye? That we must simply bracket as "noumenal." It makes no sense to ask. But about the move or "leap" from one to the other, I don't know. Can you cite something in CPR? Commented Feb 6, 2016 at 19:34
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    The noumenon/phenomenon destinction existed since his dissertation De mundi sensibilis atque intelligibilis forma et principiis from 1770 (!), the argument (shortened) is restated in his transcendental aesthetics in CPR and later improved in the Metaphysical Foundations of Natural Science from 1776, which were included in B edition of CPR. This destinction already is a priori (and, being knowledge, synthetic!). So I do not understand the wording. Perhaps reading the Prolegomena would help overcoming the problems.
    – Philip Klöcking
    Commented Feb 6, 2016 at 21:52
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    I'm with @PhilipKlöcking on this one. I don't think Kant 'makes a transition from proving the existence of synthetic apriori truths to the noumena/phenomena distinction'. In fact I don't think its clear what you mean by 'transition'.
    – M. le Fou
    Commented Feb 7, 2016 at 5:26
  • @PhilipKlöcking I am searching for a "transition" similar for example to the one Descartes uses in his cogito, where he moves from his position of radical doubt to establishing his dualism via a series of logical steps. In a couple of online lectures on Kant that I was hearing the lecturers stated that the Noumenon/Phenomenon distinction rose out of the existence of synthetic apriori, but they never bothered to give the details. (Serves me right for trying to learn from random only courses :-) ) Commented Feb 7, 2016 at 18:28
  • @M.leFou See my response to Philip Commented Feb 7, 2016 at 18:28

1 Answer 1


Kant speaks in B59f of Critique of Pure Reason (CPR) about the thing-in-itself.

The passage is an appendix to the part of CPR on Transcendental Aesthetic. The latter deals with time and space as the two forms of human intuition. Transcendental Aesthetic does not deal with synthetic a priori knowledge. The latter is constructed by the human mind by the help of the categories, see the part on Transcendental Analytics.

Things-in-themselves are noumena (B310) – the equation holds at least in first approximation.

As a consequence, the transition is introduced as a transition from intuition, the human facility to process the input of the senses, to noumena.

In B60 Kant clearly states: “The forms of space and time alone we can know a priori, that is, prior to all actual perception, and such knowledge is therefore called pure intuition. […] Even if we could impart the highest degree of distinctness to our intuition, we should not thereby come one step nearer to the constitution of object in themselves.”

How does Kant arrive at the existence of things-in-themselves? I think, that’s not difficult to understand:

It is necessary to hypothesize a source of our sense perceptions. These hypothesized objects are the things-in-themselves. But all our experience results from our processing the input of the senses by the capabilities of intuition and mind. That’s the boundary. We cannot know how the world looks before the boundary. But we can hypothesize that there are objects before the boundary.

  • Thanks. You're the only one who seems to get the gist of my question. Commented Feb 7, 2016 at 18:30

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