Introduction.
1. The basis of the physical explanation of phenomena consists in a description of the phenomena in terms of relations and of the entities subordinate to these relations. The aim of physics is to furnish the simplest explanation possible, i.e., a description employing the minimum number of entities and relations; its ideal is thus to achieve an explanation which would describe all phenomena by means of a single entity and a single relation.
Every explanation must begin with a certain number of undefined conceptions, of which it is assumed that they are generally intelligible, and that they convey the same meaning to everyone. The concept of relation itself is the first of these undefined conceptions, and so is the concept of the related entity—relatum—which is thus presupposed at the same time as the subject of the relation; both concepts are of a purely logical character.
2. Physics, as ordinarily expounded, begins by postulating two fundamental relations, space and time, which it leaves undefined, assuming their general and unique intelligibility, and to which it endeavours to reduce all other relations. At the same time with them it assumes the existence of an entity which is the subject of spatio-temporal relationships, i.e., matter, which, after space and time have been assumed, can be defined by its fundamental property of impenetrability and the inability of the same particle of matter to be in two places at the same time.
The relations of space and time, which are thus made the basis of physics, are the result of a long and complicated process of abstraction, the beginnings of which are to be found somewhere in the dawn of animal intelligence; the current conception of them, however, is very far removed from that ideal simplicity and unique intelligibility which is assumed of them by physics, and which at a first glance seems to be their characteristic; this is attested by the fact that there is no consensus of opinion even as regards their assignment into the category of relations. All who have concerned themselves with the study of the foundations of physics, are aware what irreconcilable divergencies of opinion there exist concerning these two concepts, and in what a logical circle move the majority of the attempts at their definition. The definition of space and time as “pure extension” forms an example of such a hidden circle: the definition is devoid of meaning, if by the word “extension” we do not imply “space-ness and time-ness” which implication, however, renders our definition valueless.
