Isocrates distinguished opinion from knowledge. Sometimes when we don’t know the best way to deal with something we have to use informed opinion. But we should not deceive ourselves that we are certain what the outcome will be.
χρὴ δὲ τοὺς νοῦν ἔχοντας περὶ μὲν ὧν ἴσασι μὴ βουλεύεσθαι （περίεργον γάρ） ἀλλὰ πράττειν ὡς ἐγνώκασι, περὶ ὧν δ᾽ ἂν βουλεύωνται, μὴ νομίζειν εἰδέναι τὸ συμβησόμενον, ἀλλ᾽ ὡς δόξῃ μὲν χρωμένους, ὅ τι ἂν τύχῃ δὲ γενησόμενον ἀγνοοῦντας, οὕτω διανοεῖσθαι περὶ αὐτῶν.
Intelligent men don’t debate about what they know (that’s pointless) but act according to their knowledge, but where they do have to debate they should not think that they know the outcome but treating it as an opinion, as they are ignorant of what is actually going to happen, so to think about the matter.
Isocrates Peace 8