Here’s a very brief summary of Joel Feinberg’s paper, Psychological Egoism.
Psychological Egosim (PE) is a descriptive theory of human behavior according to which self-interest is the sole motivation for all human action. People may not always know that they are acting on self-interest, but when their motivation is properly understood, it is revealed that they are acting on self-interest. People often seem to behave altruistically, but seemingly altruistic behavior can always be explained by the selfish or self-interested desires the people possess.
Reasons to accept this theory are that everyone acts on their own desires and not someone else’s, when they achieve their desires they derive pleasure, people often deceive themselves so likely do not know they are being selfish, and moral education always makes use of the pleasures and pains that certain actions lead to when teaching kids about moral conduct.
The problems with PE are that it is a non-empirical claim since such a generalization cannot be scientifically tested, the conclusion that people always act out of self-interest does not follow from the fact that people feel pleasure when their desires are achieved or that people sometimes deceive themselves about their motivations, and people who act moral only because of the rewards or punishments they will get are not wholly moral. PE seems to make wanting something in general and acting to achieve that desire selfish, but selfishness is not in the want itself, but in its object. If I want to help someone for his own benefit, that is altruistic behavior. According to the “Hedonist Paradox”, an exclusive pursuit of happiness actually keeps one from being happy. Happiness ends up coming upon those who are not looking for it. So if one has a self-interested desire for happiness, he has to pursue something other than happiness, so his interest cannot ultimately be self-interested. Finally, “pleasure” is ambiguous. It can mean a certain sensual pleasure or it can mean satisfaction of a goal that may does not involve sensual pleasure. However, it is obvious that people do not always seek pleasure in the first sense because pleasure is only a byproduct of the actions I do rather than the end. When I eat, I get pleasure from it, but I do not eat because of the pleasure, I eat to sustain myself. It cannot mean “satisfaction” because that leads to an infinite regress.