As every student in Philosophy 101 learns, nothing can force me to believe that anyone except me is conscious. This power to deny that other people have feelings is not just an academic exercise but an all-too-common vice, as we see in the long history of human cruelty.

The idea that boys want to sleep with their mothers strikes most men as the silliest thing they have ever heard. Obviously, it did not seem so to Freud, who wrote that as a boy he once had an erotic reaction to watching his mother dressing. But Freud had a wet-nurse, and may not have experienced the early intimacy that would have tipped off his perceptual system that Mrs. Freud was his mother. The Westermarck theory has out-Freuded Freud.

The more you think about and interact with other people, the more you realize that it is untenable to privilege your interests over theirs.

Commerce, trade and exchange make other people more valuable alive than dead, and mean that people try to anticipate what the other guy needs and wants. It engages the mechanisms of reciprocal altruism, as the evolutionary biologists call it, as opposed to raw dominance.

It is true that raids and battles killed a tiny percentage of the numbers that die in modern warfare. But in tribal violence, the clashes are more frequent, the percentage of men in the population who fight is greater, and the rates of death per battle are higher.

The connections I draw between human nature and political systems in my new book, for example, were prefigured in the debates during the Enlightenment and during the framing of the American Constitution.

There's a misconception that survival of the fittest means survival of the most aggressive. The adjective 'Darwinian' used to refer to ruthless competition; you used to read that in business journals. But that's not what Darwinian means to a biologist; it's whatever leads to reproductive success.