From the strict standpoint of science any theory of biological method which asserts the independence and uniqueness of the laws of biology is a vitalistic theory, and may be contrasted with the theories which deny to biology any independence of the physical sciences, the theories which we may call mechanistic. There have been many attempts, especially in recent years by the Neo-Vita-lists, to show that biology is an independent science, but these attempts have as a rule taken the line of denying that physical and chemical laws apply throughout to living organisms. They have claimed that somewhere or somehow there is a gap in the physico-chemical determinism of life, and that some vitalistic agent, psychical or metaphysical, intervenes at the gap and exerts an influence on vital processes. Thus Driesch’s Entelechy has the power of inhibiting for the proper length of time the transformation within the organism of one kind of energy into another. But it seems clear that to admit a psychical or metaphysical influence upon the physical is in effect to deny the absolute validity of physical laws, for on this admission every one of them could be altered by a non-physical agent. Any attempt then to reconcile the vitalistic theories which admit an interaction of the physical and the psychical with the prevailing mechanistic conceptions of the physical sciences is doomed to failure. One may be an adherent of Neo― Vitalism, or an adherent of the mechanistic theory; one cannot hold both views.