For Kant, as we have seen, the drive for total, systematic knowledge in reason can only be fulfilled with assumptions that empirical observation cannot support. The metaphysical facts about the ultimate nature of things in themselves must remain a mystery to us because of the spatiotemporal constraints on sensibility. When we think about the nature of things in themselves or the ultimate ground of the empirical world, Kant has argued that we are still constrained to think through the categories, we cannot think otherwise, but we can have no knowledge because sensation provides our concepts with no content. So, reason is put at odds with itself because it is constrained by the limits of its transcendental structure, but it seeks to have complete knowledge that would take it beyond those limits.
Freedom plays a central role in Kant’s ethics because the possibility of moral judgments presupposes it. Freedom is an idea of reason that serves an indispensable practical function. Without the assumption of freedom, reason cannot act. If we think of ourselves as completely causally determined, and not as uncaused causes ourselves, then any attempt to conceive of a rule that prescribes the means by which some end can be achieved is pointless. I cannot both think of myself as entirely subject to causal law and as being able to act according to the conception of a principle that gives guidance to my will. We cannot help but think of our actions as the result of an uncaused cause if we are to act at all and employ reason to accomplish ends and understand the world.
So reason has an unavoidable interest in thinking of itself as free. That is, theoretical reason cannot demonstrate freedom, but practical reason must assume for the purpose of action. Having the ability to make judgments and apply reason puts us outside that system of causally necessitated events. “Reason creates for itself the idea of a spontaneity that can, on its own, start to act–without, i.e., needing to be preceded by another cause by means of which it is determined to action in turn, according to the law of causal connection,” Kant says. (A 533/B 561) In its intellectual domain, reason must think of itself as free.
It is dissatisfying that he cannot demonstrate freedom; nevertheless, it comes as no surprise that we must think of ourselves as free. In a sense, Kant is agreeing with the common sense view that how I choose to act makes a difference in how I actually act. Even if it were possible to give a predictive empirical account of why I act as I do, say on the grounds of a functionalist psychological theory, those considerations would mean nothing to me in my deliberations. When I make a decision about what to do, about which car to buy, for instance, the mechanism at work in my nervous system makes no difference to me. I still have to peruse Consumer Reports, consider my options, reflect on my needs, and decide on the basis of the application of general principles. My first person perspective is unavoidable, hence the deliberative, intellectual process of choice is unavoidable.