What defines Sanity?

I love to play around with the idea of ultimate Sanity of human beings. What defines what is "sane" or "insane", a rebellion of conformity? What is Sanity!? It's almost satisfying to know that no one can ever really define or FULLY understand that implied meaning. Almost like the complexity of grasping eternity. Is there a reason that we cannot truly comprehend the essense of eternity? or is it a coincidence? The thought of eternity can easily push a person into insanity.