The subject of Aristotle ethical treatises is the good life for human beings. The Aristotelian 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.

The Human Function

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

This seems almost trivially true, as Aristotle himself notes. (Cf. Plato, Euthydemus 278e-279a.) The problem is to identify the activity or activities in which this good consists. Aristotle approaches this problem in terms of the "function" (ἔργον) peculiar to human beings. He argues that this function and hence that happiness consists in an exercise of reason.

• the good for human beings is happiness
• human beings are happy to the extent they exercise of their function well
• the human function is a certain exercise of reason

The use 'happiness' to translate εὐδαιμονία in Aristotle can be misleading. Aristotle is not talking about a feeling.

"The multitude and persons of refinement speak of [the good at which politics aim] as happiness, and conceive of living well (εὖ ζῆν) and doing well (εὖ πράττειν) as the same thing as being happy (εὐδαιμονεῖν). But what constitutes happiness is a matter of dispute..." (Nicomachean Ethics I.4.1095a).

"To say that the supreme good is happiness (εὐδαιμονίαν) will probably appear a truism; we still require a more explicit account of what constitutes happiness. Perhaps then we may arrive at this by ascertaining what is the function (ἔργον) of a human being" (Nicomachean Ethics I.7.1097b).

"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 Proper Virtues

If the good for a human being consists in performing the human function, and if things perform their function well when they have their proper "virtue" (ἀρετή) or "virtues" (ἀρεταί), Aristotle concludes that 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 the end among them."

• things perform their functions well when they have their proper virtues

"If we declare that the function (ἔργον) of man is a certain life, and that this is an activity and business of the soul with reason (ψυχῆς ἐνέργειαν καὶ πράξεις μετὰ λόγου), and that the good of man is to do this well (εὖ) and beautifully (καλῶς), and that if a function is completed well when it is completed in accordance with its proper virtue (οἰκείαν ἀρετὴν), then from these premises it follows that 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, then the good of man is the activity of his soul in conformity with the best and most the end among these virtues (κατὰ τὴν ἀρίστην καὶ τελειοτάτην)" (Nicomachean Ethics I.7.1098a).

Plato has Socrates set out a similar view in Book I of the Republic.

  "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).

Virtues of Thought and Character

Given the good life is one in which the part of the soul with reason has its proper virtue or virtues, it is necessary to identify this virtue or virtues.

According to Aristotle, the part of the soul with reason into two parts: a part with reason and a part capable of being controlled by reason. (Aristotle further divides the part with reason into two parts: a part that reasons about theoretical matters and a part that reasons about practical matters.) The part with reason and part capable of being controlled by reason have different proper virtues.

• the virtues proper to the human function are virtues of "thought" and "character"

"If we should say that this part [= the part capable of being controlled by reason] has reason, then the part that 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).

The virtues of character (courage, temperance, and so on) involve reasoning about practical matters. These virtues are states in which human beings act in certain ways in certain circumstances on the basis of reason. Because the action is on the basis of reason, these states are not mindless habits. The virtues of character are impossible without the virtues of thought.

"There are three elements in the soul which control action and the attainment of truth: namely, sensation, intellect, and desire (αἴσθησις νοῦς ὄρεξις). Of these, sensation never originates action, as is shown by the fact that animals have sensation but are not capable of action (πράξεως) [but only reaction to stimuli of sensation]. Pursuit and avoidance in the sphere of desire correspond to affirmation and denial in the sphere of the intellect. Since virtue of character (ἠθικὴ) is a state with respect to choice, and choice (προαίρεσις) is desire informed by deliberation, it follows that 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 [in the virtue of character] is practical (πρακτική). Of thinking that is theoretical (θεωρητικῆς), not practical nor productive, the well and badly [in this thinking are in the attainment of] true and false. This is the function (ἔργον) of the whole of thinking, but of thinking that is practical, the function is truth agreeing with correct desire" (Nicomachean Ethics VI.2.1139a).

Plato has Socrates draw a similar distinction between the virtues in the discussion of education in the Republic. "One must turn one's how soul from the world of becoming until it can endure to contemplate reality and the brightest of realities, which we say is the good. ... The other so-called virtues of the soul do seem akin to those of the body. They really do not exist before and are added later by habit (ἔθεσι) and practice. But the virtue of thought (φρονῆσαι), it seems, is certainly something more divine (θειοτέρου)..." (Republic VII.518c-518e).

The Human Function and the Divine

Aristotle, like Plato, thinks that something in human beings is like the divine.

"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 (ἔννοιαν) of what is noble and divine, this itself being 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).

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

Aristotle thinks that the good life for a human being is a life of reason, that the life of "contemplation" (θεωρία) is the best form of the life of reason, and hence that this is the best life for a human being. Contemplation is the "activity" (ἐνέργεια) in accordance with virtue of the "intellect" (νοῦς), and intellect is the most divine cognition in the human function.

(The noun ἐνέργεια is formed from ἐν ("in") and ἔργον ("function" or "work"). A standard translation is 'activity.')

Perseus Digital Library: Aristotle, Nicomachean Ethics, Eudemian Ethics
Henry George Liddell, Robert Scott, A Greek-English Lexicon: ἐνέργεια, ἔργον, εὐδαιμονία, ἠθική, τέλειος