concerned, our choice will remain without consequences. If it better satisfies our metaphysical preconceptions and habits, or preferences, we shall not commit any error if by the external world we understand some common source of percepts, the action of which on cognately constituted minds gives rise to analogous states within these minds; that the se states are analogous is inferred by each of the given minds from contact with others. Such a view of the external world as a source and cause of percepts, I think, actually predominates with the majority of physicists, perhaps implicitly with the majority of them; from the point of view of physics it cannot be erroneous, while from the point of view of natural philosophy it means a certain facilitation of the processes of thought by fixing a symbol for a comparatively concrete concept. It must, however, be clearly remembered that from the point of view of physics we are making no new assumption, but that we only replace the postulate of the correspondence between the contents and relations of two minds (which must exist in both minds, if it is to be a correspondence) by the postulate that a certain part of these minds is common to both; whether “within them” or “beyond them” is a question of metaphysics, with no bearing upon physics at all.
The Concepts of Extension and Order.
8. From what we have said so far, it is clear that it is necessary to consider the space and time of physics as real to the same extent as that to which we consider as real the perceptual content of our minds, or the “external world” of physics, as defined in previous paragraphs. This means that we are not in the least concerned with their transcendental reality. Physics knows only perceptions, and physical space and time must therefore be something within these perceptions: if we did not exist, space and time could, but, as far as physics is concerned, need not, exist.
According to the theory held in the present essay, space and time are an abstraction from the perceptual content of the mind, an abstraction resting upon the fundamental logical concept of order. The concept of order presupposes the existence of entities among which it can exist as a relation; and that again assumes the existence of another relation, which is usually called the relation of externality, and for which—to avoid the too spatial implication of the word externality—in what follows we shall employ the term ‘exclusiveness’. The totality of entities which lend themselves
