Abstract

This is a subject of recurrent interest which calls for review from time to time; I believe the last word upon it can never be said. Several topical reasons might be cited for attempting to deal with it at the present time, but these could not be pursued to the relevant applications in such a short discussion. In fact the subject is obviously a difficult one to deal with at all briefly since it is so closely bound up with other large subjects—causality, space–time, and so on