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the most esaential characteriatic of life, different Bchools advocating different material elements as the ultimate principle of life. For Democritus and most of the Atomists it was a sort of subtle fire. For Di- ogenes it was a form of air. Hippo derives it from water. Others compound it ot all the elements, whilst some of the Pythagoreans explain it as a har- mony — ^foreshadowing modern mecuanical theories. Aristotle caustically remarks that all the elements ex- cept earth had obtained a vote. With him genuine scientific and philosophic treatment of the subject be- gins; and the position to which he advanced it is amon£ the finest evidences of both his encyclopedic knowledge and his metaphysical genius. His chief discussions of the topic arc to bo found in his ircpi i^vx^ and repl ^t^r yevdfftun.

For Aristotle the chief universal phenomena of life are nutrition, growth, and decay. Movement or change in the widest sense is characteristic of all life, but plants are incapable of local movement. This followB on desire, which is the outcome of sensation. Sentiency is the differentia which constitutes the sec- ond graae of life — that of the animal kingdom. The highest kind of life is mind or reat^on, exerting itself in thought or rational activity. This last properly bo- longs to man. There are not in man three really dis- tinct souls, as Plato taught. Instead, the hij^hest or rational soul contains eminently or virtually m itself the lower animal or vegetative faculties. But what is the nature of the inner reality from which vital ac- tivity issues? Is it one of the material elements? Or is it a harmony, the resultant of the balance of bodily forces and tendencies? No. The solution for Ari^ totle is to be found in his fimdamental philosophical analysis of all Bensil)le Ixiing into the two ultimate principles, matter and form. Prime Matter (nuitcria jnima) is the common passive potential element in all sensible substances; form is the determining factor. It actualises and perfects the potential element. Neither prime matter nor any corporeal form can exist apart from each other. They are called sul>stantial principles because combined they result in a being; but they are incomplete beings in themselves, incapa- ble of existing alone. To the form is due the spe- cific nature of the being, with its activities and prop- erties. It is the principle also of unity. (Sec Fokm; Matter.) For AristotJe, in the cast^ of living natural bodies the vital principle, ^wxiJ» i« the form. His doc- trine is embodied in his famous definition: ^^xi^ iartv irrtkix^'h ^P^V (ffifJuiTOi 0i;<r(4(ou dvvdfiei ^V fx^^^^*» (De Anima, II, i), i. e. the soul is therefore the first en- telechy (substantial form or perfect actualization) of a natural or organized body potentially possessing life. The definition applies to plants, animals, and man. The human soul, however, endowed with rationality is of a higher ^rade. It is form of the body which it animates, not in virtue of its rationality but through the vegetative and sentient faculties which it also possesses. The union of these two principles is of the most intimate chfuracter, resulting in one individual being. The form, or entelochy, is therefore not a substance possessed of a distinct Ixiing from that of the body; nor in the case of animals and plants is it a reality separable from the body. The himian soul, however, seems to be of a different kind {y^yos jircpop), and separable as the eternal from the perishable. Aristotle's conception of the soul differs fundamen- tally from that of Plato for whom the vital principle is related to the body only as the pilot to the ship; who moreover distinguishes ' three numerically different soulfl in the individual man.

B, Medieval Period. — ^The Aristotelian theory in its essential features was adopted by Al))crtus Magnus and St. Thomas, and the doctrine of the vital prin- ciple as form of the body prevailed supreme through- out the Middle Ages. The differences separating the rational soul from the vital principle of the plant

or animal, and the relations between intellectual ao* tivity and sensory cognition became more clearly defined. The human soul was conceived as a spiritual substantial principle containing virtually the lower faculties of sensory and vegetative life. It is through this lower organic capacity that it is enabled to inform and animate the matter of the body. But the human soul always remains a substance capable of sulxdsting of itself apart from the body, although the operations of its lower faculties would then necessarily be sus- pended. Because of its intrinsic substantial union with the material of the organism, the two principles result in one substantial being. But since it is a spiritual being retaining spiritual activities, intrinsi- cally independent of the body, it is, as St. Thomas says, noji totalUer immersa^ not entirely submerged in matter, as are the actuating forms of the animal and the plant.

Moreover, the vital principle is the only substantial fonn of the individual being. It determines the spe- cific nature of the living being, and by the same act constitutes the prime matter with which it is imme- diately and intrinsically united a hving organized body. The Scotist School differed somewliat from this, teaching that antecedently to its union with the vital principle the organism is actuated by a certain subordinate forma corporeitutis. They conceive<.l this form or collection of forms, however, as incomplete and requiring completion by the principle of life. This conception of inferior forms, though not easy to recon- cile with the substantial unity of the human being, has never been theologically condemned, and has found favour with some modern Scholastic writers, as being helpful to explain certain biological phenomena.

With respect to the question of the origin of life Aristotle, followed by Albcrtus Magnus, St. Thomas, and the Schoolmen generally, believed in the spon- taneous generation even of organisms comparatively high in the animal kingdom (see Biogenesis). The corruption of animal and vegetable matter seemed to result in the spontaneous generation of worms and in- sects, and it was universally assumed that the earth under the influence of moisture and the sun's heat coukl produce many forms of plant and animal hfe. St. Augustine taught in the fifth century that many minute animals were not formally created on the sixth day, but only potentially in a seminal condition in certain portions of matter; and subsequently sev- eral Catholic pliilosophers and theologians admitted this view as a probable theory (cf. St. Thomas, I, Q. Ixix, a. 2: 1, Q. Ixxi, ad 1). Ilowever, the concurrent agency oi a higher cause working in nature was as- sumed as a necessarv factor by all Christian thinkers (cf . Salis Sewis, "Vera ^ottrina di S. Agostino e di S. Tom- maso contra la generazione spontanea ", Rome, 1S97).

C. Modern Period. — In respect to the nature of life, as in regard to so many other questions, Descartes (1596-1650) inaugurated a movement against the teaching of Aristotle and the Scholastics which, rein- forced by the progress of science and other influences, has during the post two centuries and a half com- manded at times considerable support among both philosophers and scientists. For Descartes there are but two agents in the universe — ^niatt^r and mind. Matter is extension; mind is thought. There is no pos- sibilitv of interaction l^etwecn them. All changes in bodies have to be explained mechanically. Vital

Erocesses such as "digestion of fowl, pulsations of eart, nutrition, and growth, follow as naturally from dispositions of the organism as the movements of a watch". Plants and animals are merely ingeniously constructed machines. Animals, in fact are merely automata. In the "Traits de I'homme" (1664), he applieft.