Those who look on physics from the outside not infrequently have the feeling that it has forgotten some of its philosophical foundations. Even among its own workers this condition of the science has not entirely escaped notice. The physicist, who, above all other men, has to deal with space and time, has fallen into certain conventions concerning them Of which he is often not aware. It may be true that these conventions are just the ones which he should make. It is certain, however, that they should be made only by one who is fully conscious of their nature as conventions and does not look upon them as fixed realities beyond the power of the investigator to modify.