One of Kant’s central contributions to philosophy was the invention of the notion of “critical philosophy,” which means epistemology, which means philosophy as a critique of knowledge. Philosophy is the queen of the sciences that polices the borders between sciences and keeps everyone safely in his or her own department. But, as Caputo points out, this is a massive concession: “Philosophy concerns a higher epistemological theory of science, but it has abandoned the real world to the sciences.”

For Kant, to be human is to be rational, and in the three critiques he maps out the domains of knowledge – theoretical knowledge, practical duty, and aesthetic beauty. Caputo helpfully explains, “By a critique he meant something cartographical (Cartesian!), staking off the boundaries of the various domains of knowledge, ethics, and art, setting off their limits, and making sure everyone plays in bounds. The result of this kind of rigorous border drawing was to isolate pure and value-free knowledge, a pure ethical command or imperative that had no content beyond its purely rational-imperative form, and pure art, devoid of cognitive or ethical content.”

For Kant, “God and religion do not have their own island, their own domain or space of playing field; they must build their house of worship on someone else’s property.” God does not belong to the sphere of knowledge because knowledge comes through sciences and God doesn’t appear there. Kant doesn’t disbelieve in God. Rather, he posits a God who is “a kind of common and more or less self-evident datum of human intelligence, liberated from any particularism, any dogmatic or confessional theology. This is a God and a theology that anyone with a head on his shoulders could understand, which is basically the God revealed in the order of nature and the God who supports the moral order.”

According to Caputo, Kant leaves two jobs for God to do. The first is a “regulative” function. The world appears to be governed by a wise ruler. This is not a matter of knowledge, but in the sphere of judgment, a “useful heuristic device, a fruitful ‘as if,’” one that “would stimulate research and advance the cause of science.” The second role is a moral one. We all face unconditional imperatives, duties that we need to accept against all desire and against our own happiness: “A religious person is someone who understands that imperative as a command of God, a God who sees to it that in the end doing your duty and happiness, which in and of themselves run on separate tracks, end up at the same station.” Religion is “ethics; it is doing your duty where the voice of duty or conscience is taken as the voice of God.” All the rest of religion is superstition. Religion is ethics, and sacraments, worship, dogma, liturgy, hymns and so on are all superstition. Reason tells us that the content of religion is only ethics understood as God’s voice speaking to us. If religions aimed at this goal, they would all agree: Dogmatically, there is going to be disagreement; but there is universal ethical consensus among religions.

Kant wanted to make room for faith by showing the limits of reason. But the faith that he made room for is rather frail, and the 19th-century critiques of religion knocked it off.