To have synthesised Newton's two great principles--his law of motion and law of gravitation--interpreting in the process the empirical law of equality of gravitational and inertial mass, is alone an immense achievement; but Einstein's researches have opened up a new world to the physicist and philosopher which is of greater importance. He has given us a vision of the immaterial world, a geometrical or mathematical vision, which is more satisfying than the "ether" conceptions hitherto presented. The fabric of his vision is not baseless. It is this fabric we shall consider, touching on certain aspects of the Einstein theory in the endeavor to present an image in miniature of his edifice of thought and to show the firmness of its foundations. That they are well and truly laid was demonstrated by the verification, from observations made during the solar eclipse in 1919, of Einstein's prediction of the displacement of a wave of light in a gravitational field, showing light to have the property of weight. The physical world is shown by Einstein to be a world of "relations." Underlying it there is an absolute world of which physical phenomena are the manifestation. "Give me matter and motion," says Descartes, "and I will construct the world." "Give me a world in which there are ordered relations," says the Relativist, "and I will show you the behavior of matter therein" (mechanics). We first view this underlying world as an abstraction, abstracting energy ("bound" as in matter and electrons, "free" as in light), and its attribute force. This abstraction we will call the "World-Frame." Later, we will study the underlying world in connection with energy, and will call this absolute world the "World-Fabric." The connection between the geometrical character of the World-Frame and the geometrical characters of the World-Fabric is the key to the law of gravitation. THE WORLD-FRAME