"The thing in itself." And that is a concept that is hard to grasp in all its glory. It refers to both a priori and empirical knowledge. The best treatments of this are from Schopenhauer and Dr Kelley Ross.
The way I see it is that Aristotle is not that far away from this concept with Prime matter being something that reason can not perceive.That is: not only do we have Plato showing there is a an aspect of reality hidden from reason but I think we have to include Aristotle in this also. For the simple reason that Reason perceives universals--not prime substance. The Gra hints to this also in his statement on the Hagada that everything has an open aspect and a hidden aspect.
Kant's original idea is modest: "It is easily seen that this object must be thought only as something in general = x, since outside our knowledge we have nothing which we could set over against this knowledge as corresponding to it...."[Immanuel Kant, Critique of Pure Reason translated by Norman Kemp Smith, St. Martin's Press, New York, 1965, p. 134.
Unbelievably simple.
But this goes further. We can check empirical knowledge. The way Kant would put it is: "We have something to set against it". That is we have a measuring stick. A Priori knowledge has no such measuring stick for its first axioms. Thus, "How do we know synthetic a priori?"
The "Obligations of the Heart" [The first Musar book by Ibn Pakuda] (What a name! Pakuda means an "order," or a "command") says it is by prophecy. That is all advances in knowledge are not by reason but by revelation.
The Rambam also mentions this in the Guide concerning moral knowledge.
Schopenhauer always refers to the "Ding An Sich" [the thing in itself] not the plural like Kant's dinge an sich things in themselves. And that brings us to the First Cause and also to the sub levels of contingency.
So then as Dr Ross puts it we do not think a bathtub full of computer chips can do any commutations.The Structures have to be already there for the mind to perceive anything.
The way this works are by Stephen Gould, Sapolsky, and by a recent essay I saw on a nice blog.
That is structures that stick out to perceive more than what they were designed for, {Stephen Gould}.
Neural networks and chaos. {Sapolsky}. Quantum jumping. [by "An Unmarried Man".] Dr Hoffman
The way to understand this the way I usually do is by the idea of plane of existence But a more fruitful approach could be through מרחב כיסוי the covering manifold.
That is p: C to X. That is "p" is a map from the covering manifold to the base manifold. And p^-1 maps in the opposite direction up to your C [cover]. And every point in your base {"X"} has an open area surrounding it, i.e. "U". And every curve in X that can be mapped to curves in upper manifolds U sub Alpha. All this means is you have lots of covering spaces over one manifold. That tomy way of thinking means lots of things in planes of existence that when they get "down here" become existing things.
[I mean this as metaphysics. Not as a biological process. But who knows?]
Anyway the place where Hegel comes into this is in the hierarchy of his triads which really are stages of unconditioned reality becoming reality.
You need Hegel because of this fellow The Maverick Philosopher that prime matter is not up to the job.
