ἀνάλλακτος
From LSJ
φιλοσοφώτερον καὶ σπουδαιότερον ποίησις ἱστορίας ἐστίν: ἡ μὲν γὰρ ποίησις μᾶλλον τὰ καθόλου, ἡ δ' ἱστορία τὰ καθ' ἕκαστον λέγει → poetry is something more scientific and serious than history, because poetry tends to give general truths while history gives particular facts
English (LSJ)
ον,
A unchangeable, Orph.Fr.248.8.