The subject of Aristotle ethical treatises is the good life for human beings. The corpus contains two works on ethics: the Nicomachean Ethics and the Eudemian Ethics. The titles seem to refer to Aristotle's friend (Eudemus of Rhodes) and Aristotle's son (Nicomachus). The relationship between the Eudemian Ethics and Nicomachean Ethics is uncertain. Books IV, V, and VI of the Eudemian Ethics are identical to Books V, VI, and VII of the Nicomachean Ethics. The Nicomachean Ethics appears to be the most authoritative of the two works.

Aristotle thinks of the inquiry into the good life as part of political science. He thinks that it is part of political science to know how to organize the city so that the citizens live good lives.

The Function of Human Beings

According to Aristotle, the good life for a human being is the life of "happiness" (εὐδαιμονία).

This seems trivially true, as Aristotle himself notes. The real problem is to identify the activity or activities in which human beings find happiness most of all. Aristotle approaches this problem in terms of the "function" (ἔργον) peculiar to human beings. He concludes that this function consists in the exercise of reason because this is the life characteristic of human beings.

(It is traditional to use 'function' to translate ἔργον in this context. Another translation is 'work.' )

"Life seems common even to plants, but we are seeking what is peculiar to man. So let us exclude the life of nutrition and growth. Next there would be a life of perception, but this also seems common to the horse, the ox, and every animal. The remaining possibility is some sort of practical life holding with reason (πρακτική τις τοῦ λόγον ἔχοντος)" (Nicomachean Ethics I.7.1097b-1098a).

• the good for a human being is happiness
• happiness is a matter of the human "function" or "work"
• the human function is a certain life of reason

This approach can seem unintuitive, but it is based on a natural idea. It is natural to think that what is good for a human being depends on what a human being is. Plato thought that a human being is an immortal "soul" (ψυχή) that is temporarily in a body. Given this understanding, the good life for a human being is different from how it was traditionally conceived. Plato thought that the good life for a human being is a certain kind of life of reason in which the human being resembles the natural state the soul enjoys outside the body. Aristotle's works in this same tradition.

Aristotle thought that human beings are reason most of all and that reason in human beings is what they share most of all with the unmovable first mover. The unmovable first mover is fixed in thought (in roughly the way Plato thought the disincarnate soul is fixed in "contemplation" (θεωρία) of the forms). The existence of the unmovable first mover is perfect. Human beings are happy most of all to the extent that their existence is most like the existence the unmovable first mover enjoys. Aristotle thought that this existence for human beings is a life of reason.

The Virtues Proper to the Human Function

Since the good for a human being consists in performing the human function well, and since things perform their function well when they have their proper "virtue" (ἀρετή) or "virtues" (ἀρεταί), the good life for a human being is the "the activity of his soul in conformity with virtue, or if there are several virtues, in conformity with the best and most complete among them."

• the best life is a matter of performing the human function well
• things perform their function well when they have their proper virtues

"If the function of man is a certain life, and this is an activity and business of the soul with reason (ψυχῆς ἐνέργειαν καὶ πράξεις μετὰ λόγου,), and the good of man is to do this well and rightly, and if a function is completed well when it is completed in accordance with its proper virtue (οἰκείαν ἀρετὴν), then the good of man is the activity of his soul in conformity with virtue (τὸ ἀνθρώπινον ἀγαθὸν ψυχῆς ἐνέργεια γίνεται κατ᾽ ἀρετήν), or if there are several virtues, in conformity with the best and most complete among them (κατὰ τὴν ἀρίστην καὶ τελειοτάτην)" (Nicomachean Ethics, I.7.1098a).

The soul, has it a work (ἔργον) which you couldn't accomplish with anything else in the world, as for example, management, rule, deliberation (ἐπιμελεῖσθαι καὶ ἄρχειν καὶ βουλεύεσθαι),
and the like, is there anything else than soul to which you could rightly assign these and say that they were its peculiar work?
Nothing else.
And again life? Shall we say that too is the function of the soul?
Most certainly.
And do we not also say that there is an excellence or virtue (ἀρετήν) of the soul?
We do.
Will the soul ever accomplish its own work well if deprived of its own virtue, or is this impossible?
It is impossible.
Of necessity, then, a bad soul will govern and manage things badly while the good soul will in all these things do well.
Of necessity.
And did we not agree that the excellence or virtue of soul is justice and its defect injustice?
Yes, we did" (Republic I.353d-353e).

The activity of the soul in conformity with virtue

The activity of the soul that matters to ethics is the part that has reason. Aristotle subdivides this part into a part that has reason strictly speaking and a part that is capable of being controlled by reason. Further, Aristotle subdivides the part that has reason strictly speaking into a part that reasons about theoretical matters and a part that reasons about practical matters.

• the proper virtues are virtues of character and virtues of thought

"If we should say that this part [that is persuaded by reason] has reason, then the part has reason will have two parts, one that has authority in itself, and one that listens as to a father. The distinction between virtues also reflects this difference. We say that some virtues are virtues of thought (διανοητικὰς) and that others are virtues of character (ἠθικάς). Wisdom (σοφίαν), quick comprehension, and practical wisdom (φρόνησιν) are virtues of thought. Generosity and temperance are virtues of character" (Nicomachean Ethics I.13.1103a1-7).

Virtues of Character and Virtues of Thought

The virtues of character (courage, temperance, and so on) involve reasoning about practical matters. The virtues of character are states according to which human beings act in certain ways in certain circumstances, and these ways of acting are not simply a matter of training and habituation. Aristotle understands these ways of acting as expressions of reason.

Because the virtues of character are expressions of reason, they are not possible without virtues of thought.

"Virtue of character (ἠθικὴ) is a state with respect to choice, and choice (προαίρεσις) is desire informed by deliberation. Both what issues from reason (λόγον) must be true and the desire must be correct for choice to be good, and reason must assert and desire must pursue the same things. This thinking (διάνοια) and truth is practical (πρακτική). Of thinking that is theoretical (θεωρητικῆς), not practical nor productive, the well and badly are true and false, as this is the function (ἔργον) of the whole of thinking. Of thinking that is practical, function is truth agreeing with correct desire (Nicomachean Ethics VI.2.1139a). "Of both intellectual (νοητικῶν) parts, the function is truth. So the virtues of both will be states in which each possesses truth most of all" (Nicomachean Ethics.VI.2.1139b).

The Divine Part of the Human Function

"If happiness (εὐδαιμονία) consists in activity (ἐνέργεια) in accordance with virtue (ἀρετὴν), it is reasonable that it should be activity in accordance with the highest virtue; and this will be the virtue of the best part of us. Whether then this be the intellect (νοῦς), or whatever else it is that is thought to rule and lead us by nature, and to have cognizance of what is noble and divine, either as being itself also actually divine, or as being relatively the divinest part of us, it is the activity of this part of us in accordance with the virtue proper (οἰκείαν ἀρετὴν) to it that will be perfect happiness (τελεία εὐδαιμονία); and it has been stated already that this activity is the activity of contemplation (θεωρητική)" (Nicomachean Ethics X.7.1177a12-18).

• he best life for a human being is a life of the divine part of the human function
• the divine part of the human function is the reason that "rules and leads a human being by nature"
• the reason that rules and leads a human being by nature is the "intellect" (νοῦς)
• the activity of the soul in accordance with virtue of the intellect is "contemplation" (θεωρία)

Aristotle thinks that the good life for a human being is a life of reason, and he thinks that this life has two forms. The life of "contemplation" (θεωρία) is the best form of the life of reason and hence the best life for a human being. Contemplation is the "activity" (ἐνέργεια) in accordance with virtue of the "intellect" (νοῦς), and intellect is the cognition most proper to human beings.

(The noun ἐνέργεια is formed from ἐν ("in") and ἔργον ("function" or "work"). It is sometimes translated as 'actuality' or 'actualization.' When Aristotle says that happiness is an ἐνέργεια in accordance with virtue, he means happiness is an activity in which some part of the human being is functioning in a way that is an expression of the virtue.)

(Henry George Liddell, Robert Scott, A Greek-English Lexicon: ἔργον, εὐδαιμονία, ἠθική)