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Two kinds of intellectual activity, both equally instinctive, have played a prominent part in the progress of physical science.

One is already developed in a child that, while holding an object, knows what will happen if he relinquishes his grasp. He may possibly never have had hold of the particular object before, but he nevertheless recognises something in common between the muscular sensations it calls forth and those which he has already experienced when grasping other objects that fell to the ground when his grasp was relaxed. Men like Galileo and Carnot, who possessed this power of perceiving analogies to an extraordinary degree, have by an analogous process built up the doctrine of energy by successive generalisations, cautious as well as bold, from experimental relationships and objective realities.

In the first place they observed, or it would perhaps be better to say that everyone has observed, that not only does an object fall if it be dropped, but that once it has reached the ground it will not rise of itself. We have to pay before a lift can be made to ascend, and the more dearly the swifter and higher it rises. Of course, the real price is not a sum of money, but the external compensation given for the work done by the lift (the fall of a mass of water, the combustion of coal, chemical change in a battery). The money is only the symbol of this compensation.

This once recognised, our attention naturally turns to the question of how small the payment can be.