The molecular entelechy finds expression in what the organic chemist and the stereochemist understand by valence, that is, the static aspect of valence considered as the structural principle of a molecule. Hence it is entelechy which makes a molecule of urea [O:C:(NH₂)₂] an entirely different substance from its isomer ammonium cyanate [NH₄·O·C:N], although the material substrate of each of these molecular units consists of precisely the same number and kinds of atoms. Similarly, it is the atomic entelechy which gives to the isotopes of Strontium chemical properties different from those of the isotopes of Rubidium, although the mass and corpuscular (electronic and protonic) composition of their respective atoms are identical. It is the vital entelechy or soul, which causes a fragment cut from a Stentor to regenerate its specific protoplasmic architecture instead of the type which would be regenerated from a similar fragment cut from another ciliate such as Dileptus. In all the tridimensional units of nature, both living and non-living, the hylomorphic analysis of Aristotle recognizes an essential dualism of matter and entelechy. Hence it is not in the presence and absence of an entelechy (as Driesch contends) that living organisms differ from inorganic units. The sole difference between these two classes of units is one of autonomy and inertia. The inorganic unit is inert, not in the sense that it is destitute of energy, but in the sense that it is incapable of self-regulation and rigidly dependent upon external factors for the utilization of its own energy-content. The living unit, on the other hand, is endowed with dynamic autonomy. Though dependent, in a general way, upon environmental factors for the energy which it utilizes, nevertheless the determinate form and direction of its activity is not imposed in all its specificity by the aforesaid environmental factors. The living being possesses a certain degree of independence with respect to these external forces. It is autonomous with a special law of immanent finality or reflexive orientation, by which all the elements and energies of the living unit are made to converge upon one and the same central result, namely, the maintenance and development of the organism both in its capacity as an individual and in its capacity as the generative source of its racial type.