Polybius, Book 33 16a fragmenta incertae sedis (Full text on the Scaife viewer)
“20. As soon as the masses are compelled to love or hate people excessively, every excuse is sufficient for them to complete their plans.
21 But I worry that I might overlook the fact that the oft-cited saying applies to me: “who is the greater fool, the one who milks a male-goat or the one who holds the bucket* to catch it?”
For, I also seem, in reporting what is agreed upon as a lie and in dragging out the process to do something very similar. For this reason, it is pointless to talk about these things, unless someone also wants to write down dreams and examine the fantasies of someone who is awake.”
*koskinos here actually means “sieve”, which makes the whole process even more futile. I simplified to “bucket” to make it easier to understand…
20. Ὅτι ὅταν ἅπαξ οἱ πολλοὶ σχῶσιν ὁρμὴν πρὸς τὸ φιλεῖν ἢ μισεῖν τινας ὑπερβαλλόντως, πᾶσα πρόφασις ἱκανὴ γίνεται πρὸς τὸ συντελεῖν τὰς αὑτῶν προθέσεις.
21. Ἀλλὰ γὰρ ὀκνῶ μή ποτ᾿ εἰς τὸ περιφερόμενον ἐμπεσὼν λάθω, πότερον ὁ τὸν τράγον ἀμέλγων ἀφρονέστερος ἢ ὁ τὸ κόσκινον ὑπέχων· δοκῶ γὰρ δὴ κἀγὼ πρὸς ὁμολογουμένην ψευδολογίαν ἀκριβολογούμενος καὶ τὸν ἐπιμετροῦντα λόγον εἰσφέρων παραπλήσιόν τι ποιεῖν. διὸ καὶ μάτην τελέως περὶ τούτων λέγειν, εἰ μή τις καὶ γράφειν ἐνύπνια βούλεται καὶ θεωρεῖν ἐγρηγορότος ἐνύπνια.
Diogenianus writes on this proverb (Centuria 95.3; Cf. Mantissa Proverb., 2.68)
“Who is the greater fool, the one who milks a male-goat or the one who holds the bucket to catch it? You should say the [one who milks] the male-goat”
Πότερον ὁ τὸν τράγον ἀμέλγων ἀφρονέστερος, ἢ ὁ τὸ κόσκινον ὑποτιθείς; εἴποις, ὁ τὸν τράγον:
Arsenius, Centuria 17 41a7
“To milk a he-goat”: this is applied to those who do something incongruous and ignorant. From this we also get the saying from Diogenianus: “Who is the greater fool, the one who milks a male-goat or the one who holds the bucket to catch it?”
“Τράγον ἀμέλγειν: ἐπὶ τῶν ἀνάρμοστόν τι ποιούντων καὶ ἀνόητον. ὅθεν καὶ Διογενιανός· πότερον ὁ τὸν τράγον ἀμέλγων ἢ ὁ τὸ κόσκινον ὑποτιθεὶς ἀφρονέστερος;