7. The Ideas of Reason

The faculty of reason has two employments. For the most part, we have engaged in an analysis of theoretical reason which has determined the limits and requirements of the employment of the faculty of reason to obtain knowledge. Theoretical reason, Kant says, makes it possible to cognize what is. But reason has its practical employment in determining what ought to be as well. (A 633/B 661) This distinction roughly corresponds to the two philosophical enterprises of metaphysics and ethics. Reason's practical use is manifest in the regulative function of certain concepts that we must think with regard to the world, even though we can have no knowledge of them.

Kant believes that, "Human reason is by its nature architectonic." (A 474/B 502). That is, reason thinks of all cognitions as belonging to a unified and organized system. Reason is our faculty of making inferences and of identifying the grounds behind every truth. It allows us to move from the particular and contingent to the global and universal. I infer that "Caius is mortal" from the fact that "Caius is a man" and the universal claim, "All men are mortal." In this fashion, reason seeks higher and higher levels of generality in order to explain the way things are. In a different kind of example, the biologist's classification of every living thing into a kingdom, phylum, class, order, family, genus, and species, illustrates reason's ambition to subsume the world into an ordered, unified system. The entire empirical world, Kant argues, must be conceived of by reason as causally necessitated (as we saw in the Analogies). We must connect, "one state with a previous state upon which the state follows according to a rule." Each cause, and each cause's cause, and each additional ascending cause must itself have a cause. Reason generates this hierarchy that combines to provide the mind with a conception of a whole system of nature. Kant believes that it is part of the function of reason to strive for a complete, determinate understanding of the natural world. But our analysis of theoretical reason has made it clear that we can never have knowledge of the totality of things because we cannot have the requisite sensations of the totality, hence one of the necessary conditions of knowledge is not met. Nevertheless, reason seeks a state of rest from the regression of conditioned, empirical judgments in some unconditioned ground that can complete the series (A 584/B 612). Reason's structure pushes us to accept certain ideas of reason that allow completion of its striving for unity. We must assume the ideas of God, freedom, and immortality, Kant says, not as objects of knowledge, but as practical necessities for the employment of reason in the realm where we can have knowledge. By denying the possibility of knowledge of these ideas, yet arguing for their role in the system of reason, Kant had to, "annul knowledge in order to make room for faith." (B xxx).