3 But let us admit that, as is said, Jupiter has himself appointed against himself ways and means by which his own declared purposes might fittingly be opposed: are we also to believe that a deity of so great majesty was dragged down to earth, and, standing on a petty hillock with a mannikin, entered into a wrangling dispute? And what, I ask, was the charm which forced Jupiter to leave the all-important direction of the universe, and appear at the bidding of mortals?
The sacrificial meal, incense, blood, the scent of burning laurel-boughs, and muttering of spells? And were all these more powerful than Jupiter, so that they compelled him to do unwillingly what was enjoined, or to give himself up of his own accord to their crafty tricks? What! will what follows be believed, that the son of Saturn had so little foresight, that he either proposed terms by the ambiguity of which he was himself ensnared, or did not know what was going to happen, how the craft and cunning of a mortal would overreach him?
You shall make expiation, he says, with a head when thunderbolts have fallen. The phrase is still incomplete, and the meaning is not fully expressed and defined; for it was necessarily right to know whether Diespiter ordains that this expiation be effected with the head of a wether, a sow, an ox, or any other animal. Now, as he had not yet fixed this specifically, and his decision was still uncertain and not yet determined, how could Numa know that Jupiter would say the head of a man, so as to anticipate and prevent him, and turn his uncertain and ambiguous words into “an onion's head?”
Source: Against the Heathen (New Advent)