Understanding provides one form of unity in our experience – let us say, at least for the sake of illustration, that form of unity constituted by the capacity to assign any given experiences a uniquely determined place relative to any other given experiences in the ideal chronology of our experience as a whole. But the unity of experience does not, as Kant sees things, exhaust the forms of unity we must seek among our representations. In addition to the unity of experience sought by understanding, Kant suggests, the faculty of reason aims at “the unity of reason” (A 302/B 359). But what might Kant mean by the unity of reason? Two ways to interpret this phrase readily come to mind. First, we might take it to imply that reason has a single domain of application: there is only one species of pure reason. Second, we might take it to connote the uniqueness of the way in which pure reason functions or the product it aims to yield: pure reason aims to introduce a single special sort of unity into whatever it is to which it is appropriately applied.

Neither of these propositions would initially appear to capture Kant's conception of pure reason. First, Kant seems to suppose, at least in the Critique of Pure Reason, that pure reason can be both theoretical and practical; while there must obviously be some connection between theoretical and practical reason, pure reason apparently cannot be described as having a unique domain of application.