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 universals only as real predicates of individual substances. Such is Aristotle’s realism of individuals and universals, contained in his primary philosophy, as expressed in the Metaphysics, especially in Book, his authoritative pronouncement on being and substance.

The individual substances, of which the universe is composed, fall into three great irreducible kinds: nature, God, man.

I. Nature.—The obvious substances are natural substances or bodies ( ), e.g. animals, plants, water, earth, moon, sun, stars. Each natural substance is a compound ( ) of essence and matter; its essence ( ) being its actual substance, its matter ( ) not; its essence being determinate, its matter not; its essence being immateriate, its matter conjoined with the essence; its essence being one in all individuals of a species, its matter different in each individual; its essence being cause of uniformity, its matter cause of accident. At the same time, matter is not nothing, but something, which, though not substance, is potentially substance; and it is either proximate to the substance, or primary; proximate, as a substance which is potentially different, e.g. wood potentially a table; primary, as an indeterminate something which is a substratum capable of becoming natural substances, of which it is always one; and it is primarily the matter of earth, water, air, fire, the four simple bodies ( ) with natural rectilineal motions in the terrestrial world (De Gen. et Cor. ii. I seq.); while aether ( ) is a fifth simple body, with natural circular motion, being the element of the stars ( ) in the celestial world. Each natural substance is a formal cause, as being what it is; a material cause, as having passive power to be changed; an efficient cause, as having active power to change, by communicating the selfsame essence into different matter so as to produce therein a homogeneous effect in the same species; and a final cause, as an end to be realized. Moreover, though each natural substance is corruptible ( ), species is eternal ( ), because there was always some individual of it to continue its original essence (expressed by the imperfect tense in  ), which is ungenerated and incorruptible; the natural world therefore is eternal; and nature is for ever aiming at an eternal propagation, by efficient acting on matter, of essence as end. For even nature does nothing in vain, but aims at final causes, which she uniformly realizes, except so far as matter by its spontaneity ( ) causes accidental effects; and the ends of nature are no form of good, nor even the good of man, but the essences of natural substances themselves, and, above them all, the good God Himself. Such is Aristotle’s natural realism, pervading his metaphysical and physical writings.

II. God.—Nature is but one kind of being (, Met. 3, 1005 a 34). Above all natural substances, the objects of natural science, there stands a supernatural substance, the object of metaphysics as theology. Nature’s boundary is the outer sphere of the fixed stars, which is eternally moved day after day in a uniform circle round the earth. Now, an actual cause is required for an actual effect. Therefore, there must be a prime mover of that prime movable, and equally eternal and uniform. That prime mover is God, who is not the creator, but the mover directly of the heavens, and indirectly through the planets of sublunary substances. But God is no mechanical mover. He moves as motive (, Met. 7, 1072 b 3); He is the efficient only as the final cause of nature. For God is a living being, eternal, very good (, ib. 1072 b 29). While nature aims at Him as design, as an end, a motive, a final cause, God’s occupation ( ) is intelligence (<span title="nóēsis"> ); and since essence, not indeed in all being, but in being understood, becomes identical with intelligence, God in understanding essence is understanding Himself; and in short, God’s intelligence is at once intelligence of Himself, of essence and of intelligence,—<span title="kaì éstin hē nóēsis noḗseōs nóēsis"> (Met. 7, 1074 b 34). But at the same time the essence of good exists not only in God and God’s intelligence on the one hand, but also on the other hand on a declining scale in nature, as both in a general and in his army; but rather in God, and more in some parts of nature than in others. Thus even God is a substance, a separate individual, whose differentiating essence is to be a living being, eternal and very good; He is however the only substance whose essence is entirely without matter and unconjoined with matter; and therefore He is a substance, not because He has or is a substratum beneath attributes, but wholly because He is a separate individual, different both from nature and men, yet the final good of the whole universe. Such is Aristotle’s theological realism without materialism and the origin of all spiritualistic realism, contained in his Metaphysics ( 6-end).

III. Man.—There is a third kind of substance, combining something both of the natural and of the divine: we men are that privileged species. Each man is a substance, like any other, only because he is a separate individual. Like any natural substance, he is composed of matter and immateriate essence. But natural substances are inorganic and organic; and a man is an organic substance composed of an organic body (<span title="organikòn sō̂ma"> ) as matter, and a soul (<span title="psychḗ"> ) as essence, which is the primary actuality of an organic body capable of life (<span title="zōḗ"> ). Still a man is not the only organism; and every organism has a soul, whose immediate organ is the spirit (<span title="pneûma"> ), a body which—analogous to a body diviner than the four so-called elements, namely the aether, the element of the stars—gives to the organism its non-terrestrial vital heat, whether it be a plant or an animal. In an ascending scale, a plant is an organism with a nutritive soul; an animal is a higher organism with a nutritive, sensitive, orectic and locomotive soul; a man is the highest organism with a nutritive, sensitive, orectic, locomotive and rational soul. What differentiates man from other natural and organic substances, and approximates him to a supernatural substance, God, is reason (<span title="lógos"> ), or intellect (<span title="noûs"> ). Now, though only one of the powers of the soul, intellect alone of these powers has no bodily organ; it alone is immortal: it alone is divine. While the soul is propagated, like any other essence, by the efficient, which is the seed, to the matter, which is the germ, of the embryo man, intellect alone enters from without (<span title="thýrathen"> ), and is alone divine (<span title="theîon">, not <span title="theós"> ), because its activity communicates with no bodily activity (De Gen. ii. 3, 736-737). A man then is a third kind of substance, like a natural substance in bodily matter, like a supernatural substance in divine reason or intellect. Such is Aristotle’s dual, or rather triple, realism, continued in his De Anima and other biological writings, especially De Generatione Animalium, ii.

There are three points about a man’s life which both connect him with, and distinguish him from, God. God’s occupation is speculative; man’s is speculation, practice and production.

1. Speculation (<span title="theōría"> ).—Since things are individuals, and there is nothing, and nothing universal, beyond them, there are two kinds of knowledge (<span title="gnō̂sis"> ), sense (<span title="aísthēsis"> ) of individuals, intellect (<span title="noûs"> ) of universals. Both powers know by being passively receptive of essence propagated by an efficient cause; but, while in sense the efficient cause is an external object (<span title="éxōthen"> ), in intelligence it is active intellect (<span title="noûs tō̂i poieîn"> ) propagating its essence in passive intellect (<span title="noûs pathētikós"> ). Nevertheless, without sense there is no knowledge. Sense receives from the external world an essence, e.g. of white, which is really universal as well as individual, but apprehends it only as individual, e.g. this white substance: intellect thereupon discovers the universal essence but only in the individuals of sense. This intellectual discovery requires sensation and retention of sensation; so that sense (<span title="aísthēsis"> ) receives impressions, imagination (<span title="phantasía"> ) retains them as images, intellect (<span title="noûs"> ) generalizes the universal, and, when it is intelligence of essence, is always true.

This is the origin of knowledge, psychologically regarded (in the De Anima). Logically regarded, the origin of all teaching and learning of an intellectual kind is a process of induction (<span title="epagōgḗ"> ) from particulars to universal, and of syllogism (<span title="syllogismós"> ) from universal to further particulars; induction, whenever it starts from sense, becomes the origin of scientific knowledge (<span title="epistḗmē"> ); while there is also a third process of example (<span title="parádeigma"> ) from particular to particular, which produces only persuasion. In acquiring scientific knowledge, syllogism cannot start from universals without induction, nor induction acquire universals without sense. At the same time, there are three species of syllogism, scientific, dialectical and eristical or sophistical; and in consequence there are different ways of acquiring premisses. In order to acquire the knowledge of the true and primary principles of scientific knowledge, and especially the