It is in its æsthetic value that the justification of the scientific theory is to be found, and with it the justification of the scientific method. Since facts without laws would be of no interest, and laws without theories would have, at most, a practical utility, we see that the motives which guide the scientific man are, from the beginning, manifestations of the æsthetic impulse. The reason why certain facts and not others interest the scientific man, the reason why he makes a choice, is because truth without beauty is as uninteresting to him as to any other artist. In the words of Poincaré: “Le savant n’étudie pas la nature parce que cela est utile; il l’étudie parce qu’il y prend plaisir, et il y prend plaisir parce qu’elle est belle. Si la nature n’était pas belle, elle ne vaudrait pas la peine d’être connue, la vie ne vaudrait pas la peine d’être vécue.” A PHYSICIST ON PHYSICS I The well-meant and industrious efforts of professional metaphysicians to explain to men of science in what sense science is true, in what sense it has meaning and in what its value really consists, practically all suffer from the defect that men of science do not recognise the subject of investigation as being science at all. It is almost true to say that the professional philosopher is only convincing when he is talking about the Absolute, for that is a subject with which nobody else is concerned; but when he devotes his attention to subjects with which other people are familiar, it often becomes possible to put the book down before finishing it. Thus treatises on æsthetics are usually convincing to everybody but poets, painters and musicians, and philosophical writings on science are probably in great demand amongst classical scholars. Nevertheless, since philosophising on these subjects is an agreeable mental exercise, we find that some artists are now engaged in developing an æsthetic for themselves, and some men of science are engaged in trying to find out what science is. In each case the work consists chiefly in making explicit processes which are instinctive. This fact is of the greatest importance, for, if the instinctive equipment be lacking, the results will inevitably be unsatisfactory. There are treatises on æsthetics, for instance, whose chief effect on the poet is to make him doubt whether the author could tell a good poem from a bad one; this is an absolutely fatal objection. If poets cannot recognise what they call poetry as being the subject of the discussion, then, as a discussion of poetry, that discussion is worthless. Practitioners, whether artists or men of science, seldom have the inclination to uncover and dissect what is to them an instinctive and delightful process; but it is quite easy for them to see (or, rather, to feel) that a suggested explanation is unsatisfactory, although they may find it wholly impossible to give reasons for their dissatisfaction.