Reason is used theoretically when it “is concerned with objects of the merely cognitive faculty” (Critique of Practical Reason, Introduction, Ak 5:14-15). The cognitive faculty is that in which objects are presented to the mind. The key problem of theoretical philosophy is the extent to which the a priori use of the cognitive faculty is legitimate: “a critical examination of it with reference to this use deals really only with the pure cognitive faculty, because the latter raised the suspicion, which was subsequently confirmed, that it might easily pass bveyond its boundaries and lose itself among unattainable objects or even among contradictory concepts” (Ak 5:15).
The theoretical use of reason is contrasted with its practical use, which is to determine the actions of the will.
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