Although the previous posts make some good points, I personally think that "I believe" is just a politically correct term intended to prevent "offending" others.  (Laurel started to say this in class.)  You either know something or you don't.  Belief should apply only to religion.  Religion is based on (for the most part) blind trust in a supernatural being. so despite arguments, no one will ever KNOW which religious follower is correct.  One religion is correct, though, whether it be the athiests, Catholics, Jews...etc.  Most people would say it should apply to ethics, since most people claim to be ethical relativists.  They say that what you "believe" is right is not necessarily right because other opinions exist.  Therefore, you do not "know" you're right--you "believe" you're right.  As an ethical objectivist, I KNOW that the highest obligation one has is to himself and his own happiness.  "Knowing" is often perceived as arrogance, so people use the word "believe" to appear more modest and "respectful" of others' beliefs.  Ironically, these people usually still think they are absolutely correct.  I don't love the words "I believe."  I hate the words "no offense."  You know what you know.  Admit it.