beLIEve!!!!

I am wondering...last saturday my friend told me about this thing..he said:'in life,we should believe each other,BUT...between believe there was a lie..'i am thinking,can we really know that people telling lie?when they telling truth o when they will telling the false?no one know that..so..please consider for a while before u beLIEve someone..if not u will regret for the whole life