D O C . 3 8 7 T H E N E W F I E L D T H E O R Y 5 7 3 of the theory of relativity. For instance, the special theory also indicated the essential identity of the conceptions’ inertial mass and energy. This is all generally known and is only mentioned here in order to emphasize the unitary tendency which dominates the whole development of the theory. THEORY OF GRAVITATION. We now turn to the second stage in the development of the theory of relativity, the so-called gen- eral theory of relativity. This theory also starts from a fact of experience which till then had re- ceived no satisfactory interpretation: the equality of inertial and gravitational mass, or, in other words, the fact known since the days of Galileo and Newton that all bodies fall with equal ac- celeration in the earth’s gravitational field. The theory uses a special theory as its basis and at the same time modifies it: the recognition that there is no state of motion whatever which is physically privileged – that is, that not only velocity but also acceleration are without absolute significance – forms the starting point of the theory. It then compels a much more profound modification of the conceptions of space and time than were involved in the special theory. For even if the special theory forced us to fuse space and time together to an invisible four-di- mensional continuum, yet the Euclidean character of the continuum remained essentially intact in this theory. In the general theory of relativity this hypothesis regarding the Euclidean charac- ter of our space-time continuum had to be abandoned and the latter given the structure of a so- called Riemannian space. Before we attempt to understand what these terms mean let us recall what this theory accomplished. It furnished an exact field theory of gravitation and brought the latter into a fully determinate relationship to the metrical properties of the continuum. The theory of gravitation, which until then had not advanced beyond Newton, was thus brought within Faraday’s conception of the field in a necessary II.- STRUCTURE OE SPACE-TIME. DUALISM OVERCOME. Professor Einstein’s second article describes, in the most general terms of which the subject ad- mits, the mathematical methods which led to the General Relativity Theory and to the new Uni- tary Field Theory. The characteristics which especially distinguish the General Theory of Relativity and even more the new third stage of the theory, the Unitary Field Theory, from other physical theories are the degree of formal speculation, the slender empirical basis, the boldness in theoretical con- struction and, finally, the fundamental reliance on the uniformity of the secrets of natural law and their accessibility to the speculative intellect. It is this feature which appears as a weakness to physicists who incline toward realism or positivism, but is especially attractive, nay fascinat- ing to the speculative mathematical mind. Meyerson in his brilliant studies on the theory of knowledge justly draws a comparison of the intellectual attitude of the relativity theoretician with that of Descartes or even of Hegel, without thereby implying the censure which a physicist would read into this.However that may be, in the end experience is the only competent judge. Yet in the meantime one thing may be said in defence of the theory. Advance in scientific knowl- edge must bring about the result that an increase in formal simplicity can only be won at the cost of an increased distance or gap between the fundamental hypothesis on the theory on the one hand and the directly observed facts on the other hand. Theory is compelled to pass more and more from the inductive to the deductive method, even though the most important demand to be made of every scientific theory will always remain: that it must fit the facts. [3] [2]