This thinking led Kant to a more pessimistic conclusion than Copernicus’s. Whereas humanity did eventually arrive at a correct understanding of the solar system, it is impossible for us to ever know “things in themselves”—what Kant called “noumena.” We have access only to “phenomena”—the way things look to us, given the kind of mind we have. “What things may be in themselves, I know not and need not know, because a thing is never presented to me otherwise than as a phenomenon,” Kant insisted.

This is an “unsettling” message, Willaschek writes: “It seems to rob all the things around us of their solidity, so to speak, and to transform them into mere figments of our imagination.” In fact, Kant didn’t intend to make us doubt the evidence of our senses. Instead, he reasoned, it is because all human beings experience the world through the same categories of time and space that scientific knowledge is possible. Science claims to deal with the world only as we perceive it, not as it is “in itself,” and to that extent it is completely reliable. Anyone who measures an object in free fall in a vacuum will find that it accelerates at thirty-two feet per second squared; we don’t have to worry that this is a “figment of our imagination.”

But Kant’s theory of knowledge poses a serious problem for any kind of religion or philosophy that claims to tell us about ultimate truths and eternal essences, such as God. If our minds are unable to reach beyond the limits of time and space, then metaphysical knowledge is a contradiction in terms. “The notion of a Supreme Being is in many respects a highly useful idea,” Kant granted, but it is only an idea. “It is incapable of enlarging our cognition with regard to the existence of things.”

Is it possible to live a meaningful existence in the absence of God and other absolute truths? This would become the central question for modern Western thought, and it was Kant who first posed it in all its complexity. The answer he offered was actually more hopeful than those of many writers who came after him. He believed that it was possible to live a good and moral life while accepting the boundaries of our understanding. But he was certain that, in philosophy as in astronomy, the “discovery of our deficiencies must produce a great change in the determination of the aims of human reason.”

Such a revolutionary ambition was fitting for a philosopher who did his most important work in the age of the American and French Revolutions. Yet in his personal life Kant was the opposite of rebellious. Willaschek organizes his book around themes—with chapters devoted to Kant’s ideas on education, revolution, wit, science, and even extraterrestrials (he believed that they must exist)—rather than chronologically, mainly because Kant’s biography is terrifically dull.

-Adam Kirsch in The New Yorker