How the special theory of relativity (see Note 4) led to the general theory of relativity (which included gravitation) may now be briefly traced. When we speak of electrons, or negative particles of electricity, in motion, we are speaking of energy in motion. Now these electrons when in motion exhibit properties that are very similar to matter in motion. Whatever deviations there are are due to the enormous velocity of these electrons, and this velocity, as has already been pointed out, is comparable to that of light; whereas before the advent of the electron, the velocity of no particles comparable to that of light had ever been measured. According to present views "all inertia of matter consists only of the inertia of the latent energy in it; ... everything that we know of the inertia of energy holds without exception for the inertia of matter." Now it is on the assumption that inertial mass and gravitational "pull" are equivalent that the mass of a body is determined by its weight. What is true of matter should be true of energy. The special theory of relativity, however, takes into account only inertia ("inertial mass") but not gravitation (gravitational pull or weight) of energy. When a body absorbs energy equation 2 (see Note 4) will record a gain in inertia but not in weight--which is contrary to one of the fundamental facts in mechanics. This means that a more general theory of relativity is required to include gravitational phenomena. Hence Einstein's General Theory of Relativity. Hence the approach to a new theory of gravitation. Hence "the setting up of a differential equation which comprises the motion of a body under the influence of both inertia and gravity, and which symbolically expresses the relativity of motions.... The differential law must always preserve the same form, irrespective of the system of coordinates to which it is referred, so that no system of coordinates enjoys a preference to any other." (For the general form of the equation and for an excellent discussion of its significance, see Freundlich's monograph, pages 27-33.) TIME, SPACE, AND GRAVITATION [14] By Prof. Albert Einstein There are several kinds of theory in physics. Most of them are constructive. These attempt to build a picture of complex phenomena out of some relatively simple proposition. The kinetic theory of gases, for instance, attempts to refer to molecular movement the mechanical thermal, and diffusional properties of gases. When we say that we understand a group of natural phenomena, we mean that we have found a constructive theory which embraces them.