The copulative verb εἶναι is often omitted, especially the forms ἐστί and εἰσί. This occurs chiefly
a. In general or proverbial statements: κοινὴ ἡ τύχη καὶ τὸ μέλλον ἀόρᾱτον
chance is common to all and the future cannot be scanned 1.29; b. in expressions of necessity, duty, etc.: ἀνάγκη φυλάττεσθαι
it is necessary to be on our guard 9.6. So with ὥρᾱ, καιρός, εἰκός, χρεών, δέον, verbals in -τέον (2152), as θεραπευτέον τοὺς θεούς
we must serve the gods M. 2.1.28; c. with various adjectives: ἄξιος, δυνατός, πρόθῡμος, δίκαιος, οἷος, φροῦδος, ἕτοιμος; thus, ἡ ψῡχὴ δουλεύειν ἑτοίμη
the soul is ready to be a servant Phae. 252a, εἴ τις ἐπερωτῴη πότερον κρεῖττον
if anybody should ask whether it is better M. 1.1.9.