if you are attacked by day or by night , fight , but retreat , without shame ; if you cross a bridge , feel every plank of it with your foot , lest one should give way beneath you ; if you pass before a house which is being built , look up , for fear a stone should fall upon your head ; if you stay out late , be always followed by your lackey , and let your lackey be armed -- if , by the by , you can be sure of your lackey .