Kant's MetaphysicsIn Germany, Kant's Critiques of Reason claimed a transcendental, non-empirical realm he called noumenal, for pure, or a priori, reason beyond or behind the phenomena. Kant's phenomenal realm is deterministic, matter governed by Newton's laws of motion. Kant's immaterial noumena are in the metaphysical non-empirical realm of the "things themselves" along with freedom, God, and immortality. Kant identified ontology not with the things themselves but, influenced by Descartes, what we can think - and reason - about the things themselves. In either case, Kant thought metaphysical knowledge might be impossible for finite minds. Kant reacted to the Enlightenment, to the Age of Reason, and to Newtonian mechanics (which he probably understood better than any other philosopher), by accepting determinism as a fact in the physical world, which he calls the phenomenal world. Kant's goal was to rescue the physical sciences from the devastating and unanswerable skepticism of David Hume, especially Hume's assertion that no number of "constant conjunctions" of cause and effect could logically prove causality. Kant called this assertion the "crux metaphysicorum." If Hume is right, he said, metaphysics is impossible. Kant's Critique of Pure Reason was to prove that Hume was wrong. Hume criticized the Theory of Ideas of his fellow British empiricists John Locke and George Berkeley. If, as they claim, knowledge is limited to perceptions of sense data, we cannot "know" anything about external objects, even our own bodies. But Hume said that we do have a natural belief in the external world and causal laws. Hume's idea of the mind having a "feeling" (not a reason) that leads to natural beliefs became Kant's "second Copernican revolution" that the mind projects "concepts of the understanding" and "forms of perception" on the external world.
Kant's main change in the second edition of the Critique of Pure Reason was an attempted refutation of this British idealism (B 274). He thought he had a proof of the existence of the external world. Kant thought it a scandal in philosophy that we must accept the existence of material things outside ourselves merely as a belief, with no proof.
The only thing which might be called an addition, though in the method of proof only, is the new refutation of psychological idealism, and the strict (and as I believe the only possible) proof of the objective reality of outer intuition. However innocent idealism may be considered with respect to the essential purposes of metaphysics (without being so in reality), it remains a scandal to philosophy, and to human reason in general, that we should have to accept the existence of things outside us (from which after all we derive the whole material for our knowledge, even for that of our inner sense) merely on trust, and have no satisfactory proof with which to counter any opponent who chooses to doubt it.Kant's noumenal world outside of space and time is a variation on Plato's concept of Soul, Descartes' mental world, and the Scholastic idea of a world in which all times are present to the eye of God. His idea of free will is a most esoteric form of compatibilism. Our decisions are made in our souls outside of time and only appear determined to our senses, which are governed by our built-in a priori forms of sensible perception, like space and time, and built-in categories or concepts of intelligible understanding. Normal | Teacher | Scholar