Like Proust, the French author whose experiences became his literary capital, man can recapture the past. He can also summon up things to come, displaying imagination and foresight along with memory. It really can be argued, that memory and foresightedness are the essence of intelligence; that man’s ability to manipulate time, to employ both past and future as guides to present action, is what makes him human.

To be sure, many animals can react to time after a fashion. A rat can learn to press a lever that will, after a delay of some 25 seconds, reward it with a bit of food. But if the delay stretches beyond 30 seconds, the animal is stumped. It can no longer associate reward so “far” in the future with present lever-pressing.

Monkeys, more smart than rats, are better able to deal with time. If one of them is allowed to see food being hidden under one of two cups, it can pick out the right cup even after 90 seconds have passed. But after that time interval, the monkey’s hunt for the food is no better than chance predicts.

With the apes, man’s nearest cousins, “time sense” takes a big step forward. Even under laboratory conditions, quite different from those they encounter in the wild, apes sometimes show remarkable ability to manipulate the present to obtain a future goal. A chimpanzee, for example, can learn stack four boxes, one on the other, as a platform from which it can reach a hanging banana. Chimpanzees, indeed, carry their ability to deal with the future to the threshold of human capacity: they can make tools. And it is by the making of tools—physical tools as crude as a stone chopper, mental tools as subtle as a mathematical equation—that man characteristically prepares for future contingencies.

Chimpanzees in the wild have been seen to strip a twig of its leaves to make a probe for extracting termites from their hole. Significantly, however, the ape does not make this tool before setting out on a termite hunt, but only when it actually sees the insects or their nest. Here, as with the banana and the crates, the ape can cope only with a future that is immediate and visible—and thus halfway into the present.