πέρθαι
From LSJ
ἐν δὲ τοῖς φυσικοῖς ἀεὶ οὕτως, ἂν μή τι ἐμποδίσῃ → in natural products the sequence is invariable, if there is no impediment | now with that which is natural it is always thus if there is no impediment
French (Bailly abrégé)
inf. ao.2 Moy. au sens Pass. de πέρθω.