Summa Theologica I-II, q. 102: Of the Causes of the Ceremonial Precepts
But if man's affection be one of passion, then it is moved also in regard to other animals: for since the passion of pity is caused by the afflictions of others; and since it happens that even irrational animals are sensible to pain, it is possible for the affection of pity to arise in a man with regard to the sufferings of animals. Now it is evident that if a man practice a pitiful affection for animals, he is all the more disposed to take pity on his fellow-men: wherefore it is written (Prov. 11:10): "The just regardeth the lives of his beasts: but the bowels of the wicked are cruel." Consequently the Lord, in order to inculcate pity to the Jewish people, who were prone to cruelty, wished them to practice pity even with regard to dumb animals, and forbade them to do certain things savoring of cruelty to animals. Hence He prohibited them to "boil a kid in the milk of its dam"; and to "muzzle the ox that treadeth out the corn"; and to slay "the dam with her young." It may, nevertheless, be also said that these prohibitions were made in hatred of idolatry. For the Egyptians held it to be wicked to allow the ox to eat of the grain while threshing the corn. Moreover certain sorcerers were wont to ensnare the mother bird with her young during incubation, and to employ them for the purpose of securing fruitfulness and good luck in bringing up children: also because it was held to be a good omen to find the mother sitting on her young.
As to the mingling of animals of divers species, the literal reason may have been threefold. The first was to show detestation for the idolatry of the Egyptians, who employed various mixtures in worshipping the planets, which produce various effects, and on various kinds of things according to their various conjunctions. The second reason was in condemnation of unnatural sins. The third reason was the entire removal of all occasions of concupiscence. Because animals of different species do not easily breed, unless this be brought about by man; and movements of lust are aroused by seeing such things. Wherefore in the Jewish traditions we find it prescribed as stated by Rabbi Moses that men shall turn away their eyes from such sights.
The figurative reason for these things is that the necessities of life should not be withdrawn from the ox that treadeth the corn, i.e. from the preacher bearing the sheaves of doctrine, as the Apostle states (1 Cor. 9:4, seqq.). Again, we should not take the dam with her young: because in certain things we have to keep the spiritual senses, i.e. the offspring, and set aside the observance of the letter, i.e. the mother, for instance, in all the ceremonies of the Law. It is also forbidden that beast of burden, i.e. any of the common people, should be allowed to engender, i.e. to have any connection, with animals of another kind, i.e. with Gentiles or Jews.
Reply to Objection 9All these minglings were forbidden in agriculture; literally, in detestation of idolatry. For the Egyptians in worshipping the stars employed various combinations of seeds, animals and garments, in order to represent the various connections of the stars. Or else all these minglings were forbidden in detestation of the unnatural vice.
They have, however, a figurative reason. For the prohibition: "Thou shalt not sow thy field with different seeds," is to be understood, in the spiritual sense, of the prohibition to sow strange doctrine in the Church, which is a spiritual vineyard. Likewise "the field," i.e. the Church, must not be sown "with different seeds," i.e. with Catholic and heretical doctrines. Neither is it allowed to plough "with an ox and an ass together"; thus a fool should not accompany a wise man in preaching, for one would hinder the other.
Reply to Objection 10[*The Reply to the Tenth Objection is lacking in the codices. The solution given here is found in some editions, and was supplied by Nicolai.] Silver and gold were reasonably forbidden (Dt. 7) not as though they were not subject to the power of man, but because, like the idols themselves, all materials out of which idols were made, were anathematized as hateful in God's sight. This is clear from the same chapter, where we read further on (Dt. 7:26): "Neither shalt thou bring anything of the idol into thy house, lest thou become an anathema like it." Another reason was lest, by taking silver and gold, they should be led by avarice into idolatry to which the Jews were inclined. The other precept (Dt. 23) about covering up excretions, was just and becoming, both for the sake of bodily cleanliness; and in order to keep the air wholesome; and by reason of the respect due to the tabernacle of the covenant which stood in the midst of the camp, wherein the Lord was said to dwell; as is clearly set forth in the same passage, where after expressing the command, the reason thereof is at once added, to wit: "For the Lord thy God walketh in the midst of thy camp, to deliver thee, and to give up thy enemies to thee, and let thy camp be holy [i.e. clean], and let no uncleanness appear therein." The figurative reason for this precept, according to Gregory (Moral. xxxi), is that sins which are the fetid excretions of the mind should be covered over by repentance, that we may become acceptable to God, according to Ps. 31:1: "Blessed are they whose iniquities are forgiven, and whose sins are covered." Or else according to a gloss, that we should recognize the unhappy condition of human nature, and humbly cover and purify the stains of a puffed-up and proud spirit in the deep furrow of self-examination.
Reply to Objection 11Sorcerers and idolatrous priests made use, in their rites, of the bones and flesh of dead men. Wherefore, in order to extirpate the customs of idolatrous worship, the Lord commanded that the priests of inferior degree, who at fixed times served in the temple, should not "incur an uncleanness at the death" of anyone except of those who were closely related to them, viz. their father or mother, and others thus near of kin to them. But the high-priest had always to be ready for the service of the sanctuary; wherefore he was absolutely forbidden to approach the dead, however nearly related to him. They were also forbidden to marry a "harlot" or "one that has been put away," or any other than a virgin: both on account of the reverence due to the priesthood, the honor of which would seem to be tarnished by such a marriage: and for the sake of the children who would be disgraced by the mother's shame: which was most of all to be avoided when the priestly dignity was passed on from father to son. Again, they were commanded to shave neither head nor beard, and not to make incisions in their flesh, in order to exclude the rites of idolatry. For the priests of the Gentiles shaved both head and beard, wherefore it is written (Bar 6:30): "Priests sit in their temples having their garments rent, and their heads and beards shaven." Moreover, in worshipping their idols "they cut themselves with knives and lancets" (3 Kings 18:28). For this reason the priests of the Old Law were commanded to do the contrary.
The spiritual reason for these things is that priests should be entirely free from dead works, i.e. sins. And they should not shave their heads, i.e. set wisdom aside; nor should they shave their beards, i.e. set aside the perfection of wisdom; nor rend their garments or cut their flesh, i.e. they should not incur the sin of schism.