Akrasia  - Usually translated as “incontinence,” this term connotes a lack of self-control. A person exhibiting akrasia knows what good behavior consists of but lacks the self-control not to give in to physical pleasures. The concept of akrasia is significant to Aristotle, as he generally agrees with the Socratic claim that no one willingly does evil and that all wrongdoing is a result of ignorance. If the incontinent person acts wrongly in full knowledge of what is good, this poses a dilemma for Socratic ethics, which Book VII of the Ethics attempts to answer.

Arete  - Usually translated as “virtue,” this important term means something more akin to “excellence.” For the Greeks, arete can be used to refer not only to a person’s moral or intellectual virtues, but to any other kind of excellence, be it the fitness of an athlete or even the sharpness of a knife. Generally speaking, a person, animal, or thing exhibits arete when it is performing its function properly. That the Greeks use the term arete in their discussions of ethics implies a strong sense that humans have a function just as knives do, and that we become good by fulfilling this function.

Doctrine of the Mean -  Aristotle’s doctrine, stated most explicitly in Book II, that virtue is a mean state between the vicious extremes of excess and deficiency. This doctrine is left necessarily vague, as Aristotle thinks that this mean varies from person to person. Essentially, it consists of the observation that it is always possible to have too much or too little of a good thing.

Energeia  - This Greek word, which is the root of our word energy, is generally translated as “activity.” However, it is not necessarily an activity in the sense that we might understand it. For instance, Aristotle describes both happiness and contemplation as activities. In calling happiness an energeia, Aristotle contrasts it with virtue, which he considers to be a hexis, or disposition. That is, the virtues dispose us to behave in the correct manner. Actually behaving according to the virtues, however, is not itself a virtue but rather the energeia of happiness.

Ethos  - We can see that this term is the root of our word ethics. However, it is more accurately translated as “character,” which gives us an important insight to understanding the Ethics. Aristotle is not so much concerned with moralizing as he is with determining what constitutes an admirable character.

Eudaimonia  - Normally translated as “happiness,” eudaimonia also carries connotations of success and fulfillment. For the Greeks, happiness is not an inner, emotional state, but the activity, or energeia, of a successful person. The Greeks did not share our sharp distinction between the public and the private, so for them, happiness is a public matter that can be evaluated just as accurately by an observer as by the person being observed.

Hexis  - Translated as “disposition,” hexis is the term Aristotle uses to qualify the virtues. According to Aristotle, virtue is not something one actively does. Rather, virtue is a disposition to behave in the right way.

Phronesis  - Often translated as “prudence,” this term is perhaps better, but more cumbersomely, translated as “practical wisdom.” Phronesis is an important intellectual virtue that allows us to reason properly about practical matters. Phronesis consists in no small part of an appropriate application of the practical syllogism.

Practical syllogism -  A syllogism is a three-term argument consisting of a major premise stating some universal truth (e.g., “All horses have four legs”), a minor premise stating some particular truth (e.g., “Black Beauty is a horse”), and a conclusion derived from these two premises (e.g., “Therefore, Black Beauty has four legs”). The practical syllogism is a form of practical reasoning in syllogistic form, the conclusion of which is an action. An example might be that the major premise “All intruders will be shot on sight” and the minor premise “I see an intruder over there” leads to the practical conclusion of shooting at the intruder.

Psuche  - The root of our word psychology, psuche is generally translated as “soul,” though it carries none of the spiritual connotations of the Christian use of that word. Psuche is that unobservable property that distinguishes living things from nonliving things. The human psuche consists of three major parts: the nutritive part, which it shares with both plants and animals; the appetitive part, which it shares with only animals; and the rational part, which is distinctively human.

Telos  - This important term can be translated variously as “end,” “goal,” or “purpose.” According to Aristotle, we have a telos as humans, which it is our goal to fulfill. This telos is based on our uniquely human capacity for rational thought. Aristotle’s view of humans having a telos based in our rationality leads directly to his conclusion in Book X that contemplation is the highest human good.