After undergoing a succession of kaleidoscopic changes, theoretical physics appears to have attained a state of comparative quiescence, in which there is fairly general agreement about essentials. In the following pages I have tried to depict the present situation in broad outline and in the simplest possible terms. I have drawn my picture against a roughly sketched background of rudimentary philosophy–the philosophy of a scientist, not of a metaphysician–because I believe, in common with most scientific workers, that without a background of this kind we can neither see our new knowledge as a consistent whole, nor appreciate its significance to the full. Statements made without reference to such a background–as, for instance, that “an electron consists of waves of probability” or that “the principle of indeterminacy shews that nature is not deterministic”–can convey at best only a minute fraction of the truth.

I have tried to exhibit the new knowledge in such a way that every reader can form his own judgment as to its philosophical implications. There is room for much legitimate difference of opinion as to what precisely these are; yet few, I think, will be found to doubt that some reorientation of scientific thought is called for.