κατανοητέον
From LSJ
τὸ γὰρ πράττειν τοῦ λέγειν καὶ χειροτονεῖν ὕστερον ὂν τῇ τάξει, πρότερον τῇ δυνάμει καὶ κρεῖττόν ἐστιν (Demosthenes 3.15) → for action, even though posterior in the order of events to speaking and voting, is prior in importance and superior
English (LSJ)
A one must observe, learn, Pl.Plt.305c, Porph.Marc.27; one must consider, πάλιν περί τινος Ph.1.83.
Greek (Liddell-Scott)
κατανοητέον: ῥημ. ἐπίθ., πρέπει νὰ μάθῃ τις, νὰ παρατηρήσῃ, Πλάτ. Πολιτ. 305C.
Dutch (Woordenboekgrieks.nl)
κατανοητέον, adj. verb. van κατανοέω, men moet beseffen.