Driesch’s definition is as follows: “Entelechy is an agent _sui generis_, non-material and non-spatial, but acting ‘into’ space.” (_Op. cit._, p. 204.) Aristotle’s use of the term in this connection is quite different. He uses it, for example, in a static, rather than a dynamic, sense: “The term ‘entelechy,’” he says, “is used in two senses; in one it answers to knowledge, in the other to the exercise of knowledge. Clearly in this case it is analogous to knowledge.” (“Peri Psyches,” Bk. II, c. 1.) Knowledge, however, is only a _second_ or static _entelechy_. Hence, in order to narrow the sense still further Aristotle refers to the _soul_ as a _first_ entelechy, by which he designates a purely _entitive_ principle, that is, a constituent of being or substance (cf. _op. cit._ _ibidem_). The _first_, or entitive, entelechy, therefore, is to be distinguished from all secondary entelechies, whether of the _dynamic_ order corresponding to kinetic energy or force, or of the _static_ order corresponding to potential energy. Neither is it an _agent_, because it is only a partial constituent of the total agent, that is, of the total active being or substance. Hence, generally speaking, _that which acts_ (the agent) is not entelechy, but the total composite of entelechy and matter, _first entelechy_ being consubstantial with matter and not a separate existent or being. In fine, according to Aristotelian philosophy, entelechy (that is, “first” or “prime” entelechy) is not an agent nor an energy nor a force. In other words, it is totally removed from the category of efficient or active causes. The second difference between Driesch and Aristotle with respect to the use of the term entelechy lies in the fact that Driesch uses it as a synonym for the soul or vital principle, whereas, according to Aristotle, _entelechy is common to the non-living units of inorganic nature as well as the living units_ (organisms) _of the organic world_. All vital principles or souls are entelechies, but not all entelechies are vital principles. All material beings or substances, whether living or lifeless, are reducible, in the last analysis, to two consubstantial principles or complementary constituents, namely, entelechy and matter. Entelechy is the binding, type-determining principle, the source of unification and specification, which makes of a given natural unit (such as a molecule or a protozoan) a single and determinate whole. Matter is the determinable and potentially-multiple element, the principle of divisibility and quantification, which can enter indifferently into the composition of this or that natural unit, and which owes its actual unity and specificity to the entelechy which here and now informs it. It is entelechy which makes a chemical element distinct from its isobare, a chemical compound distinct from its isomer, a paramœcium distinct from an amœba, a maple distinct from an oak, and a bear distinct from a tiger.