somewhat broader meaning than that of mere sense-data simplified and schematised by a process of apprehending; percept in our use of the term must comprise also hypothetical entities, such as atoms, electrons, ether waves, etc., as long as by these names we understand genuine objects of possible sense-experience, unapprehended directly because of the imperfection of our senses, but inferable from sense-experience as a whole, and not physical fictions, serving as useful pseudo-perceptual symbols of certain mathematical expressions of natural processes. We must, of course, not forget that the idea of perception, even as currently understood, is very complex, and represents the result of a complicated activity of simplification, generalisation, intention, imagination, etc., to which it is subjected before it emerges from the rough and formless sense-data, in which it “enters” consciousness; these details, however, belong to the domain of psychology and logic, and although important also for the present study, they are not essential to it, and would lead too far, if we were to occupy ourselves with them.
6. Perception presupposes the existence of a percipient, of a mind; the necessity of distinguishing the perceptual contents of the mind from its unperceptual contents then leads to the postulate of a greater number of percipients—of minds—and thus to the pre-supposition of the so-called “external world”: the denial of this external world leads to a denial also of plurality of minds—solipsism, which philosophy endeavours to avoid for reasons mainly ethical, and which physics cannot accept if it does not wish to deprive itself of the most useful criterion of perceptuality.
Speculations concerning the ontology of the external world, its mode of existence, or the manner in which it can act upon the mind, are metaphysical speculations, entirely irrelevant to physics. Some of them are of vital importance to ethics, none of them are of vital importance to physics; the latter, as we remarked above, measures and arranges its material—percepts—irrespective of whether they are the result of the action of some real existence “in itself,” or the mere product of a mind which supposes that it apprehends.
These speculations, I conjecture, do not condition even the possibility of philosophy of the physical science, the aim of which is to determine the fundamental concepts of physics and the field of its jurisdiction; from ontological speculations physics does not need to take over any but those conclusions upon which all metaphysical systems are agreed, with the exception of absolute scepticism, which
