Newton's great discovery regarding the motion of the planets consisted in his showing that these could all be summed up in the following statement: consider any planet in its relation to all particles in the universe. Write down, for the planet, in the line joining it to any particle, an acceleration proportional to the mass of the particle and to the inverse square of its distance from the planet. Then calculate the planet's resultant acceleration by combining all the accelerations thus obtained. We have here purposely avoided the use of the word "force," for Newton's law is complete as a practical statement of fact without it; and this word adds nothing to the law by way of enhancing its power in actual use. Nevertheless, the fact that the acceleration is made up as it were of non-interfering contributions from each particle in the line joining it to the planet strongly suggests to the mind something of the nature of an elastic pull for which the particle is responsible, and to which the planet's departure from a straight-line motion is due. The mind likes to think of the elastic; ever since the time of Newton people have sought to devise some mechanism by which these pulls might be visualized as responsible for the phenomena in the same way as one pictures an elastic thread as controlling the motion of a stone which swings around at its end. This search has been always without success; and now Einstein has found a rather different law which fits the facts better than Newton's law. It is of such a type that it does not lend itself conveniently to expression in terms of force; the mind would gain nothing by trying to picture such forces as are necessary. It compensates for this, however, in being capable of visualization in terms of what is ultimately a much simpler concept.