How did Immanuel Kant try to reconcile human freedom with causal determinism?

Immanuel Kant attempted to reconcile human freedom with causal determinism by distinguishing between two realms: the phenomenal and the noumenal.12

In the phenomenal world—the world as it appears to us in space and time—Kant accepted that human actions are subject to the laws of causal determinism. However, he argued that this phenomenal world is not the entirety of reality. He postulated a noumenal realm, or the world of "things-in-themselves," which is beyond our sensory experience and not governed by the same causal laws.123

Kant maintained that human beings are "inwardly free" as moral agents. By acting in accordance with the categorical imperative—universalizing one's maxims for action—the will escapes the contingencies of the phenomenal realm. While he admitted that freedom remains a "formal" or "noumenal" concept that cannot be empirically proven, he argued that it is a necessary postulate for practical reason and moral agency. Thus, humans exist as both phenomenal beings subject to necessity and noumenal beings capable of autonomy.12