Before our era,there were more than billions of generations of people.They lived before us.So we don't face them,and neither did they.But their deeds we know detailedly in the books they left us.From these,we can know how they lived,how they dealed with the troubles of the life.Because of these too,we can judge what they did is right or wrong,then lead us to do with the troubles of our morden life.Their experience is more or less our teacher.Since they silently taught us to live,it's no reasons for us to make mistakes that they made.If we make mistakes that an ancient person did,how foolish we are!Take "The Qi person worries the sky will fall" for example,someone who live nowadays worries the same thing as he,what use our morden scinence has!So the mistakes the people before made are our teacher.Therefore,I promise only make mistake you never made!