Sigmund Freud was the father of psychoanalysis, but—contrary to much apocryphal lore that dies hard—certainly not the originator of the hypothesis that unconscious ideation is essential to explain much of human overt behavior.

The generic doctrine of an unconscious domain of the mind has a venerable, long pre-Freudian history. Indeed, many of the most important doctrines commonly credited to Freud as his creations were tenets of his intellectual patrimony. Thus, as we recall from Plato's dialogue The Meno, Plato was concerned to understand how an ignorant slave boy could have arrived at geometric truths under mere questioning by an interlocutor with reference to a diagram. Plato argued that the slave boy had not acquired such geometric knowledge during his life. Instead, he explained, the boy was tapping prenatal but unconsciously stored knowledge, and restoring it to his conscious memory.

At the turn...