It seemed to Aristotle that the individual's freedom of choice made an absolutely accurate analysis of human affairs impossible. Practical science, then, such as politics or ethics, was called science only by courtesy and analogy. The inherent limitations on practical science are made clear in Aristotle's concepts of human nature and self-realization. All human beings want happiness, an active, engaged realization of their innate capacities, but this goal can be achieved in a multiplicity of ways. Aristotle's Nicomachean Ethics is an analysis of character and intelligence as they relate to happiness.
Moral virtue is an expression of character, formed by habits reflecting repeated choices.
A moral virtue is always a mean between two less desirable extremes. Intellectual virtues are not subject to this doctrine of the mean. Aristotle did not regard politics as a study of ideal states in some abstract form, but rather as an examination of the way in which ideals, laws, customs, and property interrelate in actual cases. He thus approved the contemporary institution of slavery but tempered his acceptance by insisting that masters should not abuse their authority, since the interests of master and slave are the same. The Lyceum library contained a collection of 158 constitutions of the Greek and other states.