While my reasoning is not likely to have influence on the opinions of those who really believe in altruism, for the aim of the current section, I shall pretend that psychological egoism really exists, meaning that people are always have motives to act for their own best interest. To be more specific, I shall suppose that the self-interest they are trying to achieve is nothing more then pleasure (avoidance of pain). I suppose that the pleasures that for us, humans, can be found at three levels. This idea was investigated and briefly discussed by Mill. He talks about lower level of pleasure (mostly physical) and higher level (intellectual) of pleasure. And I would like to expand them to involve one more level of pleasure, the third one, which I am going to call the Ultimate Pleasure.
No doubt, people really enjoy physical (carnal) pleasures. Eating, sleeping, touching, sexual activities and so on, are the instances of carnal pleasures. Of course, there exist plenty of physical pains we would like to escape. Some physical pains we agree to bear for a short time promise long-term benefits. All these belong to carnal pleasures, and thus this kind of pleasure is naturally one type of self-interest we all are in search for.
Another type of self-interest appears in the shape of intellectual pleasure. Acquiring knowledge, feeling loved, having a positive self-image and many other falls into the class of intellectual pleasures. The opposite for each of these components are pains that we would like to avoid all the ways possible. One seems to be happier when he can get more of these intellectual pleasures then when he is not able to. It also obvious that a person can be more happy and for a longer time by getting intellectual pleasures more then just by experiencing merely physical ones. To say in other words, physical pleasures are mostly short-lived, while intellectual pleasures can give a person much bigger and over-all pleasure and satisfaction.
Now goes, I believe, the highest level of pleasure than both the carnal and the intellectual pleasures all together can give. In other words, it is a kind of pleasure that when achieved, gives a greater strength, and much longer duration than the other two kinds. I called it the Ultimate Pleasure. This type of pleasure, to my mind, is the happiness one feels in realizing that he has acted in the "right" way. Although I cannot give an explicit definition to the "right" thing. Psychological Egoism clears up why a person chooses to act the way he acts, but makes no try to demarcate the boundaries of morally right or wrong things. So, I would better leave it up to some ethical theorists to define what is "right» in the meaning I used here. I guess for now, it is enough to say that when a person does something «right”, he wins a satisfaction that is nothing compared to all the other satisfactions possible.