If there is one question which stands forth pre-eminently from among the many problems with which physical science bristles, it is that of the ontological status of the world which physics is exploring. What is reality in the eyes of science, and what are we to understand the physicist to mean when he refers to the “real world”? Can we agree with him when he assures us that physical science represents a progress towards pure truth? There seems to be a certain reluctance on the part of twentieth-century physicists to face the ontological question which I think their colleagues of the nineteenth century did not share to the same extent.