It would be an impertinence for an amateur in physiological science to assert that the significance of ethical values can be understood only through a study of the mechanics of the human body, were not the ethical implications of physiology so numerous, so compelling, and so plainly apparent. Ever since anatomists and physiologists first began to demonstrate that all the vital functions of man were dependent upon his intimate structure; and more recently that conduct and thought are in the strictest sense of the term functions of man’s flesh, they have been laying the foundations,—even if unconsciously,—of a scrupulously natural science of ethics. The purpose of this book is to attempt the formulation of such a science. There is little need to state that any proposal to deal with ethics in a thoroughly naturalistic manner will be met with considerable resistance. Although it has been everlastingly recognized that human conduct is the direct product of human bodies, especially as is evidenced by our bestowal of rewards and punishments on individually specified persons, nevertheless, the opinion still widely prevails that a man and his actions are two such different things that the former cannot be defined in terms of the latter. Against this naturalistic or mechanistic view,—that a man _is_ what he _does_,—several classes of people tenaciously hold contradictory opinions. There are, for example, those who consider ethics as the humble handmaid of what is curiously termed “revealed religion,” and who consequently hold that all the knowledge necessary for the conduct of life has long ago been vouchsafed us by Infallible Wisdom. To such persons it is of no importance that human nature has actually altered to such an extent as to make it necessary to find new solutions even for ancient problems; nor does it seem to occur to them that the homage devoted to the past may often be simply a pleasant way of escape from the intellectual responsibilities of the present. A mechanistic ethics plans to undermine the notion that the ethical truth of one generation is necessarily sufficient for the problems of the generation that succeeds it, and aims to supersede it with the view that man’s progress is dependent, not upon his ability to escape from his problems, but rather upon his ability to analyse and solve them.