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
P. and V. ἀδύνατος, ἄπορος, ἀμήχανος (rare P.).
