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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 ?
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.
