[L]ife is a certain effort to obtain certain things from brute matter, and … instinct and intelligence, taken in their developed state, are two ways of employing an instrument for this purpose: in the first case, the instrument is part of the living creature; in the second, it is an inorganic tool, which it was necessary to invent, to fabricate and to learn to handle.

Human intelligence, as we conceive of it, is in no way the kind of intelligence depicted by Plato in the allegory of the cave. It does not have the function of watching vain shadows pass by any more than of turning round and contemplating the blazing sun. It has other things to do. Yoked, like plough-oxen, to a heavy task, we feel the play of our muscles and joints, the weight of the plough and the resistance of the soil: the function of human intelligence is to act and to know that it is acting, to enter into contact with reality and even to live it, but only in so far as it is concerned with the job being done, and the furrow being ploughed.