Like many non-specialist popularizers of psychology, Professor Sagan [in The Dragons of Eden] overestimates our physiological knowledge and underestimates our psychological knowledge. I'll get back to this point later. First, I must acknowledge that Professor Sagan has taken the first hard step in learning psychology. The first step in studying psychology is to convince yourself that there is something to study, above and beyond common sense and common knowledge. Professor Sagan likes the theory of the "triune brain," as formulated by a neurophysiologist named Paul MacLean in the early 1950's. Not an active theory in the technical literature these days, it nevertheless appeals to popularizers…. The theory depicts the human brain as combining in uneasy equilibrium our reptilian ancestry, our pre-human mammalian ancestry, and our rational, competent selves. A reptile, a mammal, and a human reason within each skin—with these wild cards, Professor Sagan can play just...