Some things are said by way of combination, others are said without combination. Thus those said by way of combination are of this sort: "man runs," "man conquers"; but those without combination are of this sort: "man," "ox," "runs," "conquers."
Of things (that are): (1) some are said of a subject, but are not in a subject; for example, "man" is said of a particular man, but is not in [the particular man]. (2) Some are in a subject but are not said of a subject. (I call "in a subject" whatever is not in something as a part yet cannot exist apart from what it is in.) For example, a particular knowledge of grammar is in a subject, the soul, but is not said of [the soul]; and a [particular] white is in a subject, body, but is not said of [body]. (3) Some are said of a subject and in a subject; for example, "knowledge" is in a subject, the soul, yet said of a particular knowledge of grammar. (4) Some are neither in a subject nor said of a subject, such as this man or this horse; for none of these things is either in a subject nor said of a subject. Certainly what is individual and numerically one is said of no subject, but nothing prevents some [of what is individual and numerically one] from being in a subject, for a particular knowledge of grammar is in a subject [yet individual and numerically one].
Whenever one thing is predicated of another as of a subject, whatever is said of the predicate also can be said of the subject. For example, "man" is said of this [particular] man, and "animal" is said of "man"; therefore "animal" will be said of this man.
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Of things said in no way by combination, each signifies either substance or a "how-much" [quantity] or a "what-kind" [quality] or a "relating-to-what" [relation] or a "where" or a "when" or a "being-in-a-position" or a "having" or a "doing" or an "undergoing." To speak roughly, substance is something like "man" or "horse"; quantity is something like "two-cubit" or "three-cubit"; quality is something like "white" or "grammatical"; relation is something like "double" or "half" or "greater"; where is something like "in the Lyceum" or "in the Agora"; when is something like "yesterday" or "last year"; being-in-a-position is something like "is lying down" or "is sitting"; having is something like "is shod" or "is armed"; doing is something like "cuts" or "burns"; undergoing is something like "is being cut" or "is being burned."
Each of the expressions above said by itself affirms nothing, but joining them to one another produces affirmation. Every affirmation is thought to be either true of false, but of things said in no way by combination none is either true of false; for example, "man," "white," "runs," "conquers."
Substance, spoken of in the strictest, primary and strongest way, is what is neither said of a subject nor in a subject; for example, this man or this horse. Secondary substances are the species of which those called primary substances are said to be; they are also the genera of these species. For example, this man is of the species "man"; the genus of this species is "animal." Therefore such things as "man" and "animal" are called secondary substances.
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All others (besides primary substances) are either said of a subject which is a primary substance, or are in a subject which is one. This becomes clear by considering an example of each. "Animal" is affirmed of "man," thus also of this man--for if not of this man, neither of "man." Again, "color" is in "body," therefore also in this body; for if not in each one [i.e., in any individual body], neither in "body." Thus everything else (besides primary substances) are either said of a subject which is a primary substance, or are in a subject which is one. Consequently, if primary substances did not exist, it is impossible for the others to exist.
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Each substance is thought to signify a this. Thus concerning primary substances, it is indisputable and true that they signify a this, for what is pointed out is individual and numerically one.
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The most peculiar trait of substances is thought to be that, [while] being [remaining] the same and numerically one, they receive contraries. That is, of the others that are not primary substances, none can be shown to be something numerically one that receives contraries. For example, a color, which is one and the same numerically, is not white and black, nor is an action, which is one numerically, both worthless and excellent; and likewise for the others which are not substances. But substance, which is something one and the same numerically, receives contraries, such as this man, being one and the same, becomes light and dark, and warm and cold, and worthless and excellent.
Since there are three [primary] substances, two physical [natural] and one changeless, concerning this [third] it must be shown that it is necessarily changeless substance. For [primary] substances are the first of beings, and if all of them are destructible, then everything is destructible.
However, it is impossible that change either comes to be or passes away (for it is everlasting); likewise with time. For no before and after could be, without the being of time [i.e., if time did not exist]. And change is as unceasing as time; [time is] either the same [as change] or a mode of change. Change is not unceasing unless it is change of place, and change of place is not unceasing unless it is cyclical.
But if something is able-to-change or able-to-do but not actually [changing or doing], then there is no change. It is possible to have a capacity yet not actualize it... It is necessary, therefore, that there be a principle ["starting-point"] such that its essence is to act [i.e., to be actual]. And further it is necessary that they be substance without material; they must be eternal, if anything else is eternal. Therefore, they are actual.
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It is clear not in reasoning alone but also in the workings of things that there is something-moved moving eternally and cyclically; thus the first heavens is everlasting. Therefore something is moving it. Since a mover that is moved is intermediate, there must be an unmoved mover, eternal and essential and actual. It is a mover like the desirable and the intelligible: moving without being moved [changing without being changed].
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Thus, that there is [a] substance eternal and unchanging and separate from the sensible is clear from what has been said.
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Book Delta, Chapter II: Explanatory Factors (Causes)
"Explanatory factor" ("cause," aition) means, first of all, the constituents out of which things are made: the bronze of a statue or the silver of a vessel, and everything of that kind. Also, there is the form or the paradigm of a thing, which is the reason or the "what-it-was-to-be," and everything of that kind (the ratio of two to one for an octave, and number generally): these are the parts of a definition. There that by means of which a change or a condition is initiated; for instance, the X; a father brings about his child, and generally, the maker bringing about what he makes, the changer bringing about change. And there is the end (telos), which is the "that-for-the-sake-of-which": walking for health, for instance. When asked, "Why are you walking?" we say, "for the sake of health," and in this way we have explained [it]. . . .
It is evident that all the explanatory factors (aitia) discussed here are of four sorts. From letters come syllables; from materials come buildings; from fire, earth, and so on come bodies; from parts come wholes; from premises come conclusions, meaning each is the explanatory factor [of the other]. Some are the "subject matter" or the "parts," some are the "what-it-was-to-be" or the "whole," the "synthesis" or the "form." A seed, a physician, a counselor or any "maker" is the cause by means of which change or change of state happens. And there is the "end" or the "good" of the others; that for the sake of which they change. . . .
LET the foregoing suffice as our account of the views concerning the soul which have been handed on by our predecessors; let us now dismiss them and make as it were a completely fresh start, endeavouring to give a precise answer to the question, What is soul? i.e. to formulate the most general possible definition of it.
We are in the habit of recognizing, as one determinate kind of what is, substance, and that in several senses, (a) in the sense of matter or that which in itself is not 'a this', and (b) in the sense of form or essence, which is that precisely in virtue of which a thing is called 'a this', and thirdly (c) in the sense of that which is compounded of both (a) and (b). Now matter is potentiality, form actuality; of the latter there are two grades related to one another as e.g. knowledge to the exercise of knowledge.
Among substances are by general consent reckoned bodies and especially natural bodies; for they are the principles of all other bodies. Of natural bodies some have life in them, others not; by life we mean self-nutrition and growth (with its correlative decay). It follows that every natural body which has life in it is a substance in the sense of a composite.
But since it is also a body of such and such a kind, viz. having life, the body cannot be soul; the body is the subject or matter, not what is attributed to it. Hence the soul must be a substance in the sense of the form of a natural body having life potentially within it. But substance is actuality, and thus soul is the actuality of a body as above characterized. Now the word actuality has two senses corresponding respectively to the possession of knowledge and the actual exercise of knowledge. It is obvious that the soul is actuality in the first sense, viz. that of knowledge as possessed, for both sleeping and waking presuppose the existence of soul, and of these waking corresponds to actual knowing, sleeping to knowledge possessed but not employed, and, in the history of the individual, knowledge comes before its employment or exercise.
That is why the soul is the first grade of actuality of a natural body having life potentially in it. The body so described is a body which is organized. The parts of plants in spite of their extreme simplicity are 'organs'; e.g. the leaf serves to shelter the pericarp, the pericarp to shelter the fruit, while the roots of plants are analogous to the mouth of animals, both serving for the absorption of food. If, then, we have to give a general formula applicable to all kinds of soul, we must describe it as the first grade of actuality of a natural organized body. That is why we can wholly dismiss as unnecessary the question whether the soul and the body are one: it is as meaningless as to ask whether the wax and the shape given to it by the stamp are one, or generally the matter of a thing and that of which it is the matter. Unity has many senses (as many as 'is' has), but the most proper and fundamental sense of both is the relation of an actuality to that of which it is the actuality. We have now given an answer to the question, What is soul?-an answer which applies to it in its full extent. It is substance in the sense which corresponds to the definitive formula of a thing's essence. That means that it is 'the essential whatness' of a body of the character just assigned. Suppose that what is literally an 'organ', like an axe, were a natural body, its 'essential whatness', would have been its essence, and so its soul; if this disappeared from it, it would have ceased to be an axe, except in name. As it is, it is just an axe; it wants the character which is required to make its whatness or formulable essence a soul; for that, it would have had to be a natural body of a particular kind, viz. one having in itself the power of setting itself in movement and arresting itself. Next, apply this doctrine in the case of the 'parts' of the living body. Suppose that the eye were an animal-sight would have been its soul, for sight is the substance or essence of the eye which corresponds to the formula, the eye being merely the matter of seeing; when seeing is removed the eye is no longer an eye, except in name-it is no more a real eye than the eye of a statue or of a painted figure. We must now extend our consideration from the 'parts' to the whole living body; for what the departmental sense is to the bodily part which is its organ, that the whole faculty of sense is to the whole sensitive body as such.
We must not understand by that which is 'potentially capable of living' what has lost the soul it had, but only what still retains it; but seeds and fruits are bodies which possess the qualification. Consequently, while waking is actuality in a sense corresponding to the cutting and the seeing, the soul is actuality in the sense corresponding to the power of sight and the power in the tool; the body corresponds to what exists in potentiality; as the pupil plus the power of sight constitutes the eye, so the soul plus the body constitutes the animal.
From this it indubitably follows that the soul is inseparable from its body, or at any rate that certain parts of it are (if it has parts) for the actuality of some of them is nothing but the actualities of their bodily parts. Yet some may be separable because they are not the actualities of any body at all. Further, we have no light on the problem whether the soul may not be the actuality of its body in the sense in which the sailor is the actuality of the ship.
This must suffice as our sketch or outline determination of the nature of soul.