To treat of the relation between life and matter, after the problem has been discussed from so many points of view and in the light of the teachings of philosophers of all ages, must seem perhaps ambitious and certainly presumptuous; but I presume that philosophers do not consider that they have completely solved the age-long problem, and some may be willing to hear what a physicist has to suggest in the light of our present knowledge of nature. I have no wish to challenge controversy, but controversial topics cannot be wholly avoided.