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How did Kant explain human freedom alongside natural causation?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — metaphysics Detail of a Roman copy (2nd century bce) of a Greek alabaster portrait bust of Aristotle (c. 325 bce); in the collection of the Museo Nazionale Romano, Rome.

Immanuel Kant addressed the tension between human freedom and natural causation by distinguishing between two realms: the phenomenal and the noumenal.1

In the phenomenal world—the world as it appears to us—Kant accepted that human actions are subject to the laws of causality and determinism. However, he argued that this phenomenal realm does not encompass the entirety of reality. Behind these appearances lies a noumenal realm of "things in themselves," which is inaccessible to speculative reason but necessary for moral agency.12

Kant explained freedom as the autonomy of the will. When an individual acts according to the categorical imperative—universalizing their maxims of action—they act as a rational agent independent of empirical desires or external contingencies. By doing so, the will escapes the determinism of the phenomenal world and participates in a noumenal "kingdom of ends".13

While Kant admitted that this leaves human freedom with a "formal" or "noumenal" character—meaning free outcomes cannot be guaranteed in the physical world—he maintained that practical reason requires us to postulate this freedom to make sense of moral responsibility.12