ἀρχῖον

From LSJ

πάντα χωρεῖ καὶ οὐδὲν μένει καὶ δὶς ἐς τὸν αὐτὸν ποταμὸν οὐκ ἂν ἐμβαίης → all things move and nothing remains still, and you cannot step twice into the same stream

Source

Spanish (DGE)

v. ἀρχεῖον.