Plato's middle dialogues present a somewhat different position. In the Republic, we find a moral psychology more complex than psychological eudaimonism: we do not only desire our ultimate good; rather the soul, or mind, has three motivating parts - a rational, spirited (approximately, emotional), and appetitive part - and each of these parts has its own desired ends. Eudaimonia, then, is not simply acquired through knowledge, it requires the correct psychic ordering of this tripartite soul: the rational part must govern the spirited and appetitive part, thereby correctly leading all desires and actions to eudaimonia and the principal constituent of eudaimonia, virtue.
Aristotle claims that "every art and every scientific inquiry, and similarly every action and purpose, may be said to aim at some good. Hence 'the good' has been well defined as that at which all things aim." According to Aristotle, the hierarchy of human purposes aim at eudaimonia as the highest, most inclusive end. This is the end that everyone in fact aims at, and it is the only end towards which it is worth undertaking means. Eudaimonia is constituted, according to Aristotle, not by honor, or wealth, or power, but by rational activity in accordance with virtue over a complete life. Such activity manifests the virtues of character, including, honesty, pride, friendliness, and wittiness; the intellectual virtues, such as rationality in judgment; and non-sacrificial (i.e. mutually beneficial) friendships and scientific knowledge (knowledge of things that are fundamental and/or unchanging is the best).
Epicurus agrees with Aristotle that happiness (eudaimonia) is the highest good. However, unlike Aristotle, he identifies happiness with pleasure. Epicurus presents two main arguments. The first defends the claim that pleasure is the only thing that people do, as a matter of fact, value for its own sake. The second, which fits in well with Epicurus' empiricism, supposedly lies in one's introspective experience: one immediately perceives that pleasure is good and that pain is bad, in the same way that one immediately perceives that fire is hot. Thus, as something immediately apparent, no further argument is needed to show the goodness of pleasure or the badness of pain. Although all pleasures are good and all pains evil, Epicurus does not believe that all pleasures are choiceworthy or all pains unchoiceworthy. Instead, one should calculate what is in one's long-term self-interest, and forgo what will bring pleasure in the short-term if doing so will ultimately lead to pain in the long-term.
The Stoics believe that virtue is necessary and sufficient for eudaimonia. Virtue consists of living according to Nature, so eudaimonia is achieved by living according to Nature's will. However, no one (except the fictional "sage") can ever achieve perfect virtue, and everyone is always equally and completely vicious. The best a non-sage can do is to act "befittingly," or in the same way that a perfectly virtuous sage would in the same situation. No one will ever achieve perfect virtue, and therefore eudaimonia, but one can approach it.