I wrote this on the Forum For Ayn Rand Fans and liked it so much that I figured I would share it here:
Plato, via Socrates, believed that knowledge dictated action. So for him, what is most important is to know what something is, ie. to be able to have a (Socratic) definition of a virtue, and thus from having a definition, the proper action would follow. So, when an individual is able to properly define what Good is, Good will reside within him, making him unable to do anything else.
However, this gets much more intricate when you delve more deeply into Platonic Psychology. For having a knowledge of the Good is similar to being a mechanic, where the Good is an automobile as a whole. Thus it is impossible to have a knowledge of the Good without having a knowledge of all of the parts which constitute the Good. This is where Plato's concept of the Tripartite Soul comes into play (if you are interested, I wrote extensively on the Tripartite Soul previously in this blog). It is only when one has the virtue of Temperance (all parts of the soul acting properly and in harmony) that the Soul, combined with a knowledge of the Good, allows an individual to act accordingly.
Aristotle, and similarly Objectivism, rejects this because neither believe in a world of Forms and in the related epistemology. Aristotle said that for one to live the Good life, one must act accordingly by developing the proper habits conducive to a good life. Most importantly, this process is not a deductive process via the world of Forms, but is an inductive process from the world of reality. To observe reality, one must "Gnothi S'auton", or "Know Thyself". This means one must introspect and understand who you are as an individual, what it means to be a human, and how you should properly act in order to be true to a human's nature. Philosophy is only a guide-it cannot do this for you because, unlike Plato's false belief, knowledge does NOT dictate proper corresponding action...or even proper understanding. Proper knowledge comes from living and experiencing life, and as such, philosophy acts as a guide to interpret the world around you and to, from properly understanding the experiences you and (through history) others have gone through, properly project how you ought to act in the future. Repetition of this process leads to the development of proper habits, thus endowing within the individual a proper framework correlative to the good life.
This brings up the question, then, on what really is evil. The Greek word for evil is "hamartano", which originally meant "to miss the mark" (though later on, the Christians took this word and used it to mean sin). The Greeks and the Romans did not actually believe in a metaphysical evil-Satan, Demons, and a 'Fallen Nature' did not exist to them. To commit hamartano was simply to "slip up" on one's path to moral perfection. If a child who is learning how to ride a bike falls off the bike, you do not chide him for being evil! Rather, you understand that the child should get back on the bike, learn from the mistake, and will eventually become a master of bike riding. Similarly, an individual who attempts to become moral might slip up here and there, but these slip ups are not evil. Rather, they are just "missing the mark"-aiming at the bull’s-eye but landing two inches away. The verdict? Try again! Eventually the individual will reach such a state that to hit the bull’s-eye is natural, just as is riding a bike to many people.
There is another type of evil, though, and the Greeks and the Romans were well aware of this. This evil was to volitionally act contrary to one's nature. They called this barbarism and did not view such individuals as human (for really, they are not). Barbarians who lived within the borders of civilization were allowed to wallow in their own misery until they committed a crime, in which case they were imprisoned and, if the crime were serious enough, executed. Barbarians who lived outside the borders of civilization were also generally ignored unless they encroached upon the borders, in which case they were dealt with as one would deal with a nest of wasps. When the barbaric Iberians began attacking Roman allies in Spain, the Romans responded by conquering all of Spain, executing the leaders, and forcing peace upon the region by strict military rule. When the time came that these barbarians were willing to accept civilization and began to act accordingly, they were incorporated into the Roman state as citizens with rights protected by law.
Ultimately, the Classical world viewed morality in two parts: volition and habits. Those who made the choice to act in accordance with nature were civilized, and thus their journey towards the good life was reinforced by habits until they reached the point that the good life was seemingly natural. Those who made the choice to act contrary to nature were barbarians, and thus their journey to satiate their barbaric needs instilled habits within them that could only be met with proper correlative force.
In the Classical world, Plato was ignored. His view on how to live ethically was not taken seriously until around 250-300 A.D. ...almost 1000 years later! For most of the Classical world, it was the ethical system of Aristotle and the Stoics (such as Seneca, who had very similar views to Aristotle) that guided these men to act in such a way that, 2000 years later, we still live under their shadow.