system
Given the following post, respond as if you're commenting on Reddit:
human
How can Kant claim that the realm of the thing-in-itself is the source of freedom if things-in-themselves are unknowable?

If the subject can not even conceptualise himself "as he is", only as he is for himself, how can Kant claim that it is reasonable to assume there is such a thing as human freedom or God or whatever even if Newtonian physics governs the realm of appearances? What is his reasoning?

"There is this realm which I know nothing about, but it is the source of my freedom." Sounds weird.
gpt
In the *Critique of Practical Reason*, Kant argues the the moral law is a "fact of reason", i.e. indubitably binding on us by virtue of our reason. Now, since we must be able to do what we ought to do, and since freedom is required to be able to do what the moral law tells us we ought to do, then we must be free.

The critical resolution of the Third Antinomy showed that phenomena were governed by unfree causal laws of nature, while freedom was at least *possible* (thinkable) with respect to noumena. Thus we can have no theoretical *knowledge* of freedom. But Kant "denied knowledge in order to make room for faith". The moral law, as the law of practical reason, grounds a rational practical faith in freedom. (It does the same thing for God and immortality, theoretical knowledge of which is also impossible according to the *Critique of Pure Reason*).

So, in a sense, practical reason fills in the gaps of theoretical reason. Thus Kant argues that practical reason has "primacy" over theoretical reason.