Psychological egoism is based completely on ones motivation that is a very private and personal matter and is sometimes attributed to the theory of selfishness. Thus, it suffers from generalization forcing it into fallacy. As psychological egoism claims that most actions are selfish, even an apparently selfless action sometimes becomes a selfish action. Ethical egoism has two versions, stronger and weaker. Although the weaker version states that it is moral to endorse one's interests but contradicts itself by saying that even if one does not endorse one's interests it is not necessarily immoral. The stronger version clears the problem, of how it is moral to endorse one's own interests and still not moral to do so, by clearly stating that it is right to follow one's self- interest and immoral not to do so. Thus, the strong version affirms that it is moral to pursue one's well being and never moral not to do so, while the weak version claims that though it is moral to pursue one's well being, the opposite is not always accurate. (Szabados, 2004)
The most basic and important difference between the two is in their principle statement.