Plato: The Idealist

The main distinction between the world of appearances and the ideal world of forms is introduced.

1) appearances are what we ordinarily take to be real, but they are really just fleeting, less-than-perfect instances of Reality that can be known only insofar as it is contemplated.

2) the world of forms contains those eternally perfect essences that the lesser instances that we experience conditionally and contingently "partake" in, and derive their reality from (more or less, but never completely).

The distinction between function and virtue is also introduced.

1) function: is what x is designed to do, what it is best suited for, or what it does best.

2) virtue: is the function of x being achieved well or with excellence.

For Plato, the human being is defined as having a tri-part soul (with intellectual, spirited, and appetitive elements). One is virtuous when her "spirit" and "appetites" (passions and biological needs) are subordinated to reason, for the wellbeing of the larger whole.

Aristotle: The Realist

The sharper dualism between ideal (form) and (unreal) appearance is reconstructed so that the form is essential to matter, giving it its particular nature. The form makes the matter what it is, more specifically.

1) form is brought down to earth as a working scientific reality, so the dualism is tempered.

2) form and matter cooperate, but are still ontologically distinct (not the same kind of "thing").

3) the distinction between soul and body depends on this philosophical restructuring of ideas.

Human beings are defined as "rational animals", and the concept of moral virtue is refined accordingly: as being passions and actions in the right amount. Our souls contain nutritive, sensitive, and rational (intellectual and moral) elements. Reason finds the means between the extremes of excess and deficiency in our passions, and this is how a moral virtue is developed. More specifically, these 3 criteria must be met:

1) knowedge: one must know what she is doing so that "doing the right thing" is no accident.

2) intention: the goodness is chosen deliberately, for its own sake or because it is good.

3) habituation: the action is done so frequently that it becomes a trait of character or "condition of the soul"

In the end moral virtues -- although excellences in themselves -- have the further aim of establishing eudaimonia or the more pervasive form of personal (intellectual and moral) wellbeing loosely translated as "happiness".

Aquinas and Augustine came to rest their theories on this largely Aristotelian concept of happiness, in developing what became principles of Roman Catholic Natural Law.