Xenophanes on our subjection to opinion (δόκος), appropriately preserved by quote in Sextus Empiricus (in Adversus Mathematicos 7.49, against the logicians). Text and translation is the Loeb Early Greek Philosophy v.3:
And thus there has never been any man, nor will there ever be one,
Who knows what is clear about the gods and whatever I say about all things.
For even if he happened most to say something perfect,
He himself nonetheless does not know: opinion is set upon all things.
καὶ τὸ μὲν οὖν σαφὲς οὔτις ἀνὴρ γένετ’ οὐδέ τις ἔσται
εἰδὼς ἀμφὶ θεῶν τε καὶ ἅσσα λέγω περὶ πάντων·
εἰ γὰρ καὶ τὰ μάλιστα τύχοι τετελεσμένον εἰπών,
αὐτὸς ὅμως οὐκ οἶδε· δόκος δ᾽ ἐπὶ πᾶσι τέτυκται.
Extracts 