Ethics is the branch of philosophy that deals with morality, or good versus bad. Most agree that people have a responsibility to act morally. Society is built on the shared trust that people must place in one another every day. Therefore, people have an obligation to others as fellow human beings, and to the society as a whole, to act morally. Ethical theories help people determine what the right course of action may be in a given situation.
People have been concerned about how to live morally for thousands of years. The Greeks, beginning with Plato, were the first to formalize the study of ethics. However, Plato's own student, Aristotle, disagreed with his teacher on how a person should live morally. Thus, the topic of ethics has never been static. After the fall of Greek civilization, it was not until Europe emerged from the Dark Ages that Western philosophy again had serious ethical debates and new ideas. However, Eastern ethics evolved separately, on its own timetable, with thinkers like Confucius.
Plato's ethical theories were the first in Western civilization. Virtue Ethics grew from his thinking. It holds that a person's character and motivations are much more important than his duties and man-made laws.
Aristotle had a different concept of ethics, namely that men were happiest and fulfilled their responsibilities best when they served the state and used their ability to reason. He also believed in the Golden Mean, or everything in moderation.
Many centuries later, Immanuel Kant created the Categorical Imperative. This requires that a person only act in ways that he thinks would be beneficial if the entire world acted the same. So, if a person wants to steal, he would have to be fine with the idea that anyone else could steal from him.
During Europe's Dark Ages, there was not much advancements in ethics. This is because much of antiquity's knowledge was lost, and many great societies that had supported thinkers disappeared. Some thinkers, like St. Thomas Aquinas, stand out, yet the Catholic Church was also part of the reason there were so few new ideas. It claimed to have absolute authority over ethics and moral responsibility. Centuries later, thinkers like Rene Descartes were spending a vast amount of time attempting to prove the existence of God and trying not to contradict the Church.
Ethics continues to evolve in modern times. In the 20th century, one of the most influential thinkers was Ayn Rand, whose philosophy of objectivism called for people to serve no one but themselves--any kind of servitude was immoral as slavery. Other philosophers, like Peter Singer, are writing today and exploring new areas, such as humans' moral responsibility to animals.