[Tim has again welcomed some of my attempts to understand philosophy, this time in its modern cast, for which I thank him.]
Kant’s Prolegomena to Any Future Metaphysics presents a helpful precìse of his “critical” philosophy, without which no one today would have any idea what Kant meant by the billowing swells of the Critique of Pure Reason. In the Prolegomena Kant is primarily concerned with the epistemological conditions for “pure philosophical cognition” (reine philosophische Erkenntnis) or Metaphysics. What is the noetic framework whereby we “judge” metaphysical concepts?
Straightaway it is necessary to point out that Kant has a very different conception of knowledge and metaphysics than classical or scholastic philosophy. Following the pathway of Descartes’ “turn to the subject,” Kant does not take the epistemological veracity of human knowledge for granted, does not assume that the mind accurately represents things as they are. For the moderns, I do not “receive” the form or logos of the sensible, a form which transcendends the “subject-object” distinction (see Aristotle’s De Anima, II). Rather, I “produce” the sensible insofar as I represent or construct it for myself based on impressions or motions it causes within me and to which it bears no formal likeness. Kant’s mind lacks access to things-in-themselves (noumena); it is restricted to things as they appear (phenomena), insofar as it constructs these appearances through its own categories. Hume had impugned the metaphysical category of causality on the grounds that it was a fiction of the mind imposed on (an untrustworthy) “reality.” For the Scotch philosopher, experience was the only source of knowledge, uncertain because not necessary. Against Aristotle and the medievals, Kant agrees with Hume that the mind constructs reality insofar as it is known by us. But Kant thinks that the structure of human epistemology can be deduced with complete certainty through the basic principles of reason (like the law of non-contradiction). The Kantian or “Copernican” revolution consists in understanding metaphysical truth/reality as fiction, starting from the mind that measures or fabricates this fiction—rather than as (ontological) truth in itself, starting from perception.