It is described as follows by Moseley (2006): "The individual aims at her own greatest good" and "it is always moral to promote one's own good, and it is never moral not to promote it." The 'should' makes it a prescriptive doctrine; in its strong version, the theory holds that pursuing self-interest is the same as being moral and that a person should let nothing stop him form reaching his long term goals - every action should lead to this. This idea is supported in Ayn Rand's philosophy of Objectivism, where she considers altruism (the opposite of egoism) to be immoral. She says in the Introduction to her book 'The Virtue of Selfishness' (Rand, 1964, 7)
The weaker version admits that not everybody does this, as self-interested actions can help others. In fact, Hobbes, in 'Leviathan' (1651) states in his Introduction that, despite having self-serving desires due to our mechanical nature, "the wealth, and riches of all the particular members are the strengths;" This suggests that values are around wealth and attaining our own ends, but Hobbes sees the man with these as contributing to the good of the whole society. The fact that ethical egoists obey laws, and carry out duties is not especially selfish, this too shows up as the weaker version.
Psychological Egoism: This is called an empirical theory because it derives from observation of human nature and doe