Shabbat
Saving a human life.  Jews learn from  that one must violate the laws of Shabbat to save a human life.  Sometimes, there are two different ways to save a life.  Then the person should try to pick the way that violates Shabbat least.  But that is not always possible.  And saving a life always comes first.  Even if someone is not sure he will be successful to save a life, trying to save life on Shabbat always comes first.