Hey, I'm a philosophy major and just finished a course on ethics, so I know alot about this... I gotsa go study for my psychology exam, but after that tomorrow I'll post... In short, my opinion is that yes, altruism can be easily defended logically in philosophical dialectic.
But, I should point out, that there are many ethical theories which bridge the gap between egoism (every agent should act in his or her own interest solely) and altruism (every agent should act in the interest of others solely). Just to clarify, I don't think that librasoul is asking about altruism as the ethical system--few people would defend that or egoism. Most people balance a bit of each and use other systems (consequentialism, Kantianism, virtue ethics (although consequentialists are accused of being almost entirely altruistic)). I think what librasoul is actually asking if purely altruistic acts can occur---and I'd say yes. More on that later....
Ack. I'm such a dork.
