Follow the progression here from desiring soul to thinking soul, and then to an account of the human function. I take from two of Aristotle's works, and from a few parts in those works.
De Anima, II.3-4Of the potentialities of the soul which have been mentioned, some existing things have them all, as we have said, others some of them, and certain of them only one. The potentialities which we mentioned are those for nutrition, sense-perception, desire, movement in respect of place, and thought.
Plants have the nutritive faculty only; other creatures have both this and the faculty of sense-perception. And if that of sense-perception, then that of desire also; for desire comprises wanting, passion, and wishing: all animals have at least one of the senses, touch, and for that which has sense-perception there is both pleasure and pain and both the pleasant and the painful: and where there are these, there is also wanting: for this is a desire for that which is pleasant.
Furthermore, they have a sense concerned with food; for touch is such a sense; for all living things are nourished by dry and moist and hot and cold things, and touch is the sense for these and only incidentally of the other objects of perception; for sound and color and smell contribute nothing to nourishment, while flavor is one of the objects of touch. Hunger and thirst are forms of wanting, hunger is wanting the dry and hot, thirst wanting the moist and cold; and flavor is, as it were, a kind of seasoning of these. We must make clear about these matters later, but for now let us say this much, that those living things which have touch also have desire....
(II.4) Anyone who is going to engage in inquiry about these [the soul-capacities] must grasp what each of them is and then proceed to investigate what follows and the rest. But if we must say what each of them is, e.g. what is the faculty of thought or of perception or of nutrition, we must again first say what thinking and perceiving are; for activities and actions are in respect of definition prior to their potentialities. And if this is so, and if again, prior to them, we should have considered their correlative objects, then we should for the same reason determine first about them, e.g. about nourishment and the objects of perception and thought.
Hence, we must first speak about nourishment and reproduction; for the nutritive soul belongs also to the other living things and is the first and most commonly possessed potentiality of the soul, in virtue of which they all have life. Its functions are reproduction and the use of food; for it is the most natural function in living things, such as are perfect and not mutilated or produced by generation, to produce another thing like themselves-- an animal to produce an animal, a plant a plant-- in order that they may partake of the everlasting and divine in so far as they can; for all desire that, and for the sake of that they do whatever they do in accordance with nature. (But that for the sake of which is twofold-- the purpose for which and the beneficiary for whom.) Since, then, they cannot share in the everlasting and divine by continuous existence, because no perishable thing can persist numerically one and the same, they share in them in so far as each can, some more and some less; and what persists is not the thing itself but something like itself, not one in number but one in species.
De Anima, III.4-5Given that the intellect is something simple and unaffected, and that it has nothing in common with anything else, as Anaxagoras says, someone might raise these questions: how will it think, if thinking is being affected in some way (for it is in so far as two things have something in common that the one is thought to act and the other to be affected)? And can it itself also be thought? For either everything else will have intellect, if it can itself be thought without this being through anything else and if what can be thought is identical in form, or it will have something mixed in it which makes it capable of being thought as the other things are.
Now, being affected in virtue of something common has been discussed before-- to the effect that the intellect is in a way potentially the objects of thought, although it is actually nothing before it thinks; potentially in the same way as there is writing on a tablet on which nothing actually written exists; that is what happens in the case of the intellect. And it is itself an object of thought, just as its objects are. For, in the case of those things which have no matter, that which thinks and that which is thought are the same; for contemplative knowledge and that which is known in that way are the same. The reason why it does not always think we must consider. In those things which have matter each of the objects of thought is present potentially. Hence,
they will not have intellect in them (for intellect is a potentiality for being such things without their matter), while
it will have what can be though in it.
(III.5) Since in the whole of nature there is something which is matter to each kind of thing (and this is what is potentially all those things), while on the other hand there is something else which is their cause and is productive by producing them all-- these being related as an art to its material-- so there must also be these differences in the soul. And there is an intellect which is of this kind by becoming all things, and there is another which is so by producing all things, as a kind of disposition, like light, does; for in a way light too makes colors which are potential into actual colors. And this intellect is distinct, unaffected, and unmixed, being in essence activity.
For that which acts is always superior to that which is affected, and the first principle to the matter. Actual knowledge is identical with its object; but potential knowledge is prior in time in the individual but not prior even in time in general; and it is not the case that it sometimes thinks and at other times not. In separation it is just what it is, and this alone is immortal and eternal. (But we do not remember because this is unaffected, whereas the passive intellect is perishable, and without this thinks nothing.)
Nicomachean Ethics, I.7Let us again return to the good we are seeking, and ask what it can be. It seems different in different actions and arts; it is different in medicine, in strategy, and in the other arts likewise. What then is the good of each? Surely that for whose sake everything else is done. In medicine this is health, in strategy victory, in architecture a house, in any other sphere something else, and in every action and choice the end; for it is for the sake of this that all men do whatever else they do. Therefore, if there is an end for all that we do, this will be the good achievable by action, and if there are more than one, these will be the goods achievable by action.
So the argument has by a different course reached the same point; but we must try to state this even more clearly. Since there are evidently more than one end, and we choose some of these (e.g. wealth, flutes, and in general instruments) for the sake of something else, clearly not all ends are complete ends; but the chief good is evidently something complete. Therefore, if there is only one complete end, this will be what we are seeking, and if there are more than one, the most complete of these will be what we are seeking. Now we call that which is in itself worthy of pursuit more complete than that which is worthy of pursuit for the sake of something else, and that which is never desirable for the sake of something else more complete than the things that are desirable both in themselves and for the sake of that other thing, and therefore we call complete without qualification that which is always desirable in itself and never for the sake of something else.
Now such a thing happiness, above all else, is held to be; for this we choose always for itself and never for the sake of something else, but honor, pleasure, reason, and every excellence we choose indeed for themselves (for if nothing resulted from them we should still choose each of them), but we choose them also for the sake of happiness, judging that through them we shall be happy. Happiness, on the other hand, no one chooses for the sake of these, nor, in general, for anything other than itself.
From the point of view of self-sufficiency the same result seems to follow; for the complete good is thought to be self-sufficient. Now by self-sufficient we do not mean that which is sufficient for a man by himself, for one who lives a solitary life, but also for parents, children, wife, and in general for his friends and fellow citizens, since man is sociable by nature. But some limit must be set to this; for if we extend our requirement to ancestors and descendants and friends' friends we are in for an infinite series. Let us examine this question, however, on another occasion; the self-sufficient we now define as that which when isolated makes life desirable and lacking in nothing; and such we think happiness to be; and further we think it most desirable of all things, without being counted as one good thing among others-- if it were so counted it would clearly be made more desirable by the addition of even the least of goods; for that which is added becomes an excess of goods, and of goods the greater is always more desirable. Happiness, then, is something complete and self-sufficient, and is the end of action.
Presumably, however, to say that happiness is the chief good seems a platitude, and a clearer account of what it is is still desired. This might perhaps be given, if we could first ascertain the function of man. For just as for a flute-player, a sculptor, or any artist, and, in general, for all things that have a function or activity, the good and the 'well' is thought to reside in the function, so would it seem to be for man, if he has a function. Have the carpenter, then, and the tanner certain functions or activities, and has man none? Is he naturally functionless? Or as eye, hand, foot, and in general each of the parts evidently has a function, may one lay it down that man similarly has a function apart from all these? What then can this be? Life seems to be common even to plants, but we are seeking what is peculiar to man. Let us exclude, therefore, the life of nutrition and growth. Next there would be a life of perception, but
it also seems to be common even to the horse, the ox, and every animal.
There remains, then, an active life of the element that has a rational principle (of this, one part has such a principle in the sense of being obedient to one, the other in the sense of possessing one and exercising thought); and as this too can be taken in two ways, we must state that life in the sense of activity is what we mean; for this seems to be the more proper sense of the term. Now if the function of man is an activity of soul in accordance with, or not without, rational principle, and if we say a so-and-so and a good so-and-so have a function which is the same in kind, e.g. a lyre-player and a good lyre-player, so without qualification in all cases, eminence in respect of excellence being added to the function (for the function of the lyre-player is to play the lyre, and that of a good lyre-player is to do so well): if this is the case, and we state the function of man to be a certain kind of life, and this to be an activity or actions of the soul implying a rational principle, and the function of a good man to be the good and noble performance of these, and if any action is well performed when it is performed in accordance with the appropriate excellence: if this is the case, human good turns out to be activity of soul in conformity with excellence, and if there are more than one excellence, in conformity with the best and most complete.
But we must add 'in a complete life'. For one swallow does not make a summer, nor does one day; and so too one day, or a short time, does not make a man blessed and happy.
Let this serve as an outline of the good; for we must presumably first sketch it roughly, and then later fill in the details....
