It is with some trepidation that I am raising again this old problem, for it may well be said that nothing new could possibly be written on it. On the other hand it can scarcely be maintained that what has been written has left the problem in a satisfactory state. Moreover, it seems to me that some work which has been done recently on the nature of theoretical physics can be used to dispose of most of the difficulties, and it is partly in the hope of drawing attention to that work and its bearings on psychology that I am writing this paper.