The faculty of imagination. The ability to visualise or conceptualise something which does not exist, and then take steps to make it exist. It is what transformed the various human species over millions of years from timid scavengers into apex predators - predators capable of killing other predators. It was a desirable characteristic that guided mating choices and thus became ever more developed.

Too developed, because then we began to imagine things we should not have done - such as how the world would be if we destroyed those Othere who were like us but not like us, and took their territory to add to ours. The roots of war, empire and genocide.

Then we began to imagine things that could never exist, could not be made to exist. So we imagined that they had always existed instead, and that they talked to some of us. Now we had kingship, and priesthood, and exploitation and inequality.

Now we imagine that there is happiness and contentment to be had in something more than good health, a full belly and a warm, safe place to sleep and raise our young. Look at that cat, lying on the sofa. It's been fed, it's out of the cold and wet, and nothing nearby can or will harm it. So it purrs and sleeps, content. That is how we should be, our bodily needs fulfilled, we should be content. But our overactive imaginations insist there must be something more, create desires which cannot be fulfilled and dangers which are no threat. So we are not content, we try instead to be 'happy', another imaginary state.

Epicurus once said that the height of pleasure was nothing more than the absence of pain. If we were less imaginative, we'd see that he was right, and we would suffer far less.