One of the strangest of the many strange habits of philosophers, which mark them out as the Ishmaels of the scientific world, is their refusal to agree as to the precise meaning of the words they use. No philosopher, it seems, is bound by the definitions given by predecessors or contemporaries of even the most central terms: each has to define his terms for himself. The resulting situation certainly lends itself to ridicule and caricature, as in the legend of the theological disputants who arrived after long argument at the conclusion that when the one said ‘God’ he meant what the other meant when he said ‘Devil.’ Still it is probable that this idiosyncrasy of philosophers has some real ground in the special nature of the task on which they are engaged, and is not a mere exhibition of aimless malice or sheer incompetence. Whether that is so or not, one of the consequences of this situation is that the titles of philosophic discourses are apt to be singularly unilluminating: as an indication of the problem to be raised they are, to say the least, highly ambiguous. How the reader may understand the title of this paper I do not know; but the question which I had in mind in choosing it was this.