The ancient eudaimonists were not misguided when they gave a prominent place to the human function in their ethical theory. Most modern reconstructions of eudaimonism do not employ the human function in this way. Though this gives them the appearance of being more streamlined and plausible, they fail to unify a life which respects the demands of justice. It is evident that in the Republic and other ancient ethical works humans are presented as acting out of concern for the good of others. They show respect for justice and act from altruistic motivation, and this is one source of value for a human life according to eudaimonism. But the question arises of whether this is the only sort of value a human life can have. Surely not; it matters as well whether a human life is flourishing and happy. How do these two aspects or areas of a human life combine? To present actions done from altruistic motivation as existing in splendid isolation from the actions done for the sake of happiness threatens to split a life and to fracture its overall structure. The better way is to propose that all actions, both those motivated by altruistic concerns and those motivated by the desire for happiness, are performed for the sake of happiness, with the caveat that the first set of actions need not be performed with any intention of achieving happiness. This respects the fact of altruistic motivation while integrating that motivation into the fabric of a happy life. But to achieve this unity, we must take seriously the notion of the human function and make room for it within the structure of eudaimonism. Only when we use the human function to develop a version of eudaimonism is it possible to describe the demands of justice as categorical, as making unconditioned demands on our practical rationality, and also to describe actions performed to meet these demands as being oriented to happiness as their end.
Payne, Andrew, "Eudaimonism and the Demands of Justice" (2006). The Society for Ancient Greek Philosophy Newsletter. 404.