" [...] Look at the way people try to make points of contact with others when they meet. Look at the way you instinctively try to establish whether somebody you meet for the first time knows somebody you know. Watch people do it. I'm from such and such place. Oh yes? So you know So-and-So? Yes! And So-and-So? No, I don't know her, but of course, there's So-and-So. That's how it works." "I suppose so. But why?" "Because we don't like impersonality. Maybe..." David joined in. "Because we had to." She asked him: "Had to what?" "Because he had to co-operate. That's deep in the genes. We had to co-operate with one another and so we needed to know whether the stranger was a threat. [...] "