ποταμῷ γὰρ οὐκ ἔστιν ἐμβῆναι δὶς τῷ αὐτῷ → it is impossible to step twice in the same river, you cannot step twice into the same rivers
τὸ, Ατο υπέρθυρο.[ΕΤΥΜΟΛ. < ὑπερ- + -θύριον (< θύρα), πρβλ. παρα-θύρι(ον)].