Le métier d’homme est difficile.

—Georges Simenon, Le neige était sale

I n his memoirs, Sir Arthur Conan Doyle says that the philosophy of Moleschott was all the rage when he was a medical student in Edinburgh during the late 1870s and early 1880s. Jacob Moleschott (1822–93) was a Dutch physiologist who had also studied philosophy in his youth and was a militant materialist. He is remembered today, if at all, for a couple of aphorisms that sum up his philosophy: “There is no thought without phosphorus” and “The brain secretes thought as the liver secretes bile.”

I suppose it is true that, metaphorically speaking, thought can become as blocked as bile, and even more irritant in its effects. But this summary, and surely very premature, dismissal of the puzzle of human...