Every animal leaves traces of what it was; man alone leaves traces of what he created. - Jacob Bronowski

Mass, time, magnetic moment, the unconscious: we have grown up with these symbolic concepts, so that we are startled to be told that man had once to create them for himself. He had indeed, and he has: for mass is not an intuition in the muscle, and time is not bought ready-made at the watchmaker's.

The genius of men like Newton and Einstein lies in that: they ask transparent, innocent questions which turn out to have catastrophic answers. The poet William Cowper called Newton a ‘childlike sage’ for that quality, and the description perfectly hits the air of surprise at the world that Einstein carried in his face. Whether he talked about riding a beam of light or falling through space, Einstein was always full of beautiful, simple illustrations of such principles, and I shall take a leaf out of his book. I go to the bottom of the clocktower, and get into the tram he used to take every day on his way to work as a clerk in the Swiss Patent Office.

Of these massive achievements I shall single out two. One is Newton’s working out of the concept of the cause, by making it over from its scholastic form in, say, St. Thomas Aquinas, to the modern form which now seems so obvious to us. This is one theme in this chapter. But I shall go to it by way of a related achievement, and to my mind one which is as remarkable: the marriage of the logical with the empirical method.