Plato is perhaps the most influential philosopher of all time, and he is widely regarded as the first truly systematic thinker in Western intellectual culture. No less a mind than the esteemed British mathematician and philosopher Alfred North Whitehead once quipped that the "safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato." While this is obviously a slight exaggeration, it is nevertheless true that Plato is the originator of several highly regarded ideas in Western philosophy, including the longest revered branch of ethical philosophical thought: virtue ethics.
Contemporary philosophers still disagree on what exactly the term "ethics" means. Many such philosophers today consider ethical language to be nothing more than a moral fiction. Nevertheless, the general consensus in the field diverges among three major branches: consequentialism, deontologicalism and virtue ethics. The first two are relatively recent ideas, but virtue ethics has been around since the time of Plato. Virtue ethics focuses on the idea that what we call good is not dependent on the actions we take (deontologicalism) nor the results of those actions (consequentialism), but instead focuses on the person that we are. To a virtue ethicist like Plato, actions are only good to the extent that virtuous persons take such actions. When Plato talks about what is good, he always means for us to think of an ideal good person. In this way, Plato would agree wholeheartedly with the basic idea of the "What Would Jesus Do?" movement since the focus is on what a good person is rather than what good actions or good consequences are.
Eudaimonia and Arete
For Plato, ethics comes down to two basic things: eudaimonia and arete. Eudaimonia, or "well being," is the virtue that Plato teaches we must all aim toward. The ideal person is the person who possesses eudaimonia, and the field of ethics is mostly just a description of what such an ideal person would truly be like. However, achieving eudaimonia requires something extra, which Plato calls arete, or excellence. Possessing arete is the way that one can reach a state of eudaimonia. A person with arete is a person who has the character traits that would lead to a eudaimonious life. If given enough time, the set of virtues will help anyone to become eudaimonious. Most of Plato's writings about ethics focuses on what arete is, with the idea that if one can figure that out, then eudaimonia will follow shortly after.
What Is Arete?
Plato's earliest ideas on arete revolve around the question whether each positive character trait we might name would be a part of arete. For example, is courage part of arete? Surely so, Plato argues, since we would hardly call a cowardly person's life eudaimonious. However, maybe courage is only an effect of a eudaimonious life and not a cause. Questions like this plague the early Plato, but by his middle period, he seems to have decided on arete being nothing more than just pure knowledge. Knowledge of all things is important, but none is more important than knowledge of knowledge itself, which Plato considers to be the ultimate virtue and a necessary component for any individual to achieve eudaimonia. Perhaps shockingly to modern readers, Plato also includes several other items as necessary conditions for eudaimonia, including luck and wealth.
Plato's Evolving Views
While Plato never strays from his conception of virtue ethics throughout his life, by the time he fully matures his views, he does seem to clarify what arete and eudaimonia are by quite a bit. By the end of his career, Plato decides that true eudaimonia is not really achievable on Earth in the same way that he thought was the case earlier on. Early Plato acts as though striving for arete is a realistic goal to have; yet later Plato questions whether arete is even possible without first possessing knowledge of the universe as a whole. For this older Plato, a person cannot even know what arete really is without knowing the form of the good itself, and so the greatest good comes from knowing the measure of one's own knowledge.