Aristotle’s dualism of matter and form, which is but a physical application of his transcendental dualism of potency (_dynamis_) and act (_entelechy_), is very different from the Cartesian dualism of the physical and the psychic. According to the Aristotelian view, as we have seen in the last chapter, all the physical entities or substantial units of nature (both living and inorganic) are fundamentally _dual_ in their essence, each consisting of a definitive principle called entelechy and a plastic principle called matter. Entelechy is the integrating determinant, the source of the unit’s coherence and of its differentiation from units of another type. Matter is the determinable and quantifying factor, in virtue of which the unit is potentially-multiple and endowed with mass. In the electro-chemical reactions of non-living substances (synthesis, analysis, and transmutation), entelechy is the variant and matter is the constant; in the metabolic activities of living substances (assimilation and dissimilation), matter is the variant and entelechy is the constant. This persistent entelechy of the living unit or organism is what Aristotle terms the _psyche_ or soul. The latter, therefore, may be defined as the vital principle or primary source of life in the organism. But in using such terms as “soul” and “vital principle” we are employing expressions against which not merely rabid mechanists, but many conservative biologists as well, see fit to protest. The opposition of the latter, however, is found on closer scrutiny to be _nominal_ rather than _real_. It is the _name_ which offends; they have no objection to the _thing signified_. Wilson, to cite a pertinent example, rejects as meaningless all such terms as “vital principle,” “soul,” etc. “They are words,” he avers, “that have been written into certain spaces that are otherwise blank in our record of knowledge, and as far as I can see no more than this.” (“Biology,” p. 23, 1908.) Yet he himself affirms again and again the existence of the reality which these terms (understood in their Aristotelian sense) denote. In discussing the relation of the tissue cell to the multicellular body, for instance, he speaks of “a formative power pervading the growing mass as a whole.” (“The Cell,” 2nd ed., p. 59), and, in his recent lecture on the “Physical Basis of Life,” he makes allusion to “the integrating and unifying principle in the vital processes.” (_Science_, March 9, 1923, p. 284.) It would seem, therefore, that Wilson’s aversion to such terms as soul and vital principle is based on the _dynamic_ sense assigned to them by the neo-vitalists, who, as we have seen, regard the vital principle as a force _sui generis_ or a _unique agent_, which operates intrusively among physicochemical factors in the rôle of an active or efficient cause of vital functions. That such is really the case, appears from his rhetorical question: “Shall we then join hands with the neo-vitalists in referring the