21 But this, too, it is fitting that we should here learn from you: If a goat be slain to Jupiter, which is usually sacrificed to father Liber and Mercury, or if the barren heifer be sacrificed to Unxia, which you give to Proserpine, by what usage and rule is it determined what crime there is in this, what wickedness or guilt has been contracted, since it makes no difference to the worship offered to the deity what animal it is with whose head the honour is paid which you owe?
It is not lawful, says my opponent, that these things should be confounded, and it is no small crime to throw the ceremonies of the rites and the mode of expiation into confusion. Explain the reason, I beg. Because it is right to consecrate victims of a certain kind to certain deities, and that certain forms of supplication should be also adopted. And what, again, is the reason that it is right to consecrate victims of a certain kind to certain deities, and that certain forms of supplication should he also adopted, for this very rightfulness should have its own cause, and spring, be derived from certain reasons?
Are you going to speak about antiquity and custom? If so, you relate to me merely the opinions of men, and the inventions of a blind creature: but I, when I request a reason to be brought forward to me, wish to hear either that something has fallen from heaven, or, which the subject rather requires, what relation Jupiter has to a bull's blood that it should be offered in sacrifice to him, not to Mercury or Liber. Or what are the natural properties of a goat, that they again should be suited to these gods, should not be adapted to the sacrifices of Jupiter?
Has a partition of the animals been made among the gods? Has some contract been made and agreed to, so that it is fitting that this one should hold himself back from the victim which belongs to that, that the other should cease to claim as his own the blood which belongs to another? Or, as envious boys, are they unwilling to allow others to have a share in enjoying the cattle presented to them? Or, as is reported to be done by races which differ greatly in manners, are the same things which by one party are considered fit for eating, rejected as food by others?
Source: Against the Heathen (New Advent)