The Western tradition of ethics began with Socrates. The Pre-Socratic thinkers were concerned with questions about the origins of the cosmos, or the basic constituent elements of reality, such as fire, earth, air and water. The Pre-Socratics represent the origins of philosophical thought because they attempted to answer their questions through reason rather than religion. Socrates made man himself the object of philosophy. The primary question that motivated Socrates was the question of the good life: its definition and how one lives it. Socrates laid the foundation for traditional ethics.
Socrates and Plato laid the foundation for ethical thinking, but Aristotle was the first one to develop a systematic ethical system. Aristotle's ethics was one of virtue. Ethical life, according to Aristotle, is integral to human achievement. Aristotle held a teleological view of the world. That is to say, everything has a specific function and purpose. In Aristotelian philosophy, the purpose of human beings is to live a life in accordance with reason, which means living in accordance with virtue. However, Aristotle distinguished between theoretical reason -- logic and science -- and practical reason. Ethics is a form of practical reason. It is not a perfect science; it confronts the ambiguities and uncertainties of life. Practical wisdom is the ability to choose the right thing in the right way at the right time, which was Aristotle's definition of virtue. This is a skill that develops over time and becomes part of our character.
Utilitarianism evaluates the morality of an act through its consequences. Jeremy Bentham and John Stuart Mill were the two central figures responsible for this philosophy. Bentham developed a moral calculus based on the "greatest happiness principle." An action is moral that produces the greatest amount of happiness for the greatest number of people. Bentham interpreted happiness in terms of pleasure and the avoidance of pain. Mill presented a more sophisticated form of Utilitarianism. He made a qualitative distinction between pleasures. He emphasized the importance of education and its role in training individuals in the higher pleasures. He was an early supporter of women's rights and equality.
Deontology proposes that the morality of an act is evaluated by its intention and not its consequences. Deontology emphasizes duty over happiness and is derived from two Greek words: "deon" (duty) and "logos" (science). Immanuel Kant was the central figure associated with deontological ethics. He argued that the only thing that is good, without qualification, is "good will." Human beings, in Kant's view, are constituted by will. Although we are natural beings who have to conform to the laws of nature, we also have the freedom to choose to act in certain ways. Kant modeled his idea of the moral law on the universality of the laws of nature. Every decision is based on a subjective principle or maxim that guides the action. An action is moral only if the maxim can be universalized; in other words, there are no exceptions. This is Kant's categorical imperative. The categorical imperative is arrived at through reason. "It's okay to tell a lie depending upon the circumstances" is an example of a maxim that cannot be universalized as it rationally contradicts itself.