Summa Theologica I-II, q. 102: Of the Causes of the Ceremonial Precepts
If, however, we speak of bodily uncleanness, consisting in some kind of corruption, the flesh of certain animals is unclean, either because like the pig they feed on unclean things; or because their life is among unclean surroundings: thus certain animals, like moles and mice and such like, live underground, whence they contract a certain unpleasant smell; or because their flesh, through being too moist or too dry, engenders corrupt humors in the human body. Hence they were forbidden to eat the flesh of flat-footed animals, i.e. animals having an uncloven hoof, on account of their earthiness; and in like manner they were forbidden to eat the flesh of animals that have many clefts in their feet, because such are very fierce and their flesh is very dry, such as the flesh of lions and the like. For the same reason they were forbidden to eat certain birds of prey the flesh of which is very dry, and certain water-fowl on account of their exceeding humidity. In like manner certain fish lacking fins and scales were prohibited on account of their excessive moisture; such as eels and the like. They were, however, allowed to eat ruminants and animals with a divided hoof, because in such animals the humors are well absorbed, and their nature well balanced: for neither are they too moist, as is indicated by the hoof; nor are they too earthly, which is shown by their having not a flat but a cloven hoof. Of fishes they were allowed to partake of the drier kinds, of which the fins and scales are an indication, because thereby the moist nature of the fish is tempered. Of birds they were allowed to eat the tamer kinds, such as hens, partridges, and the like. Another reason was detestation of idolatry: because the Gentiles, and especially the Egyptians, among whom they had grown up, offered up these forbidden animals to their idols, or employed them for the purpose of sorcery: whereas they did not eat those animals which the Jews were allowed to eat, but worshipped them as gods, or abstained, for some other motive, from eating them, as stated above (A[3], ad 2). The third reason was to prevent excessive care about food: wherefore they were allowed to eat those animals which could be procured easily and promptly.
With regard to blood and fat, they were forbidden to partake of those of any animals whatever without exception. Blood was forbidden, both in order to avoid cruelty, that they might abhor the shedding of human blood, as stated above (A[3], ad 8); and in order to shun idolatrous rite whereby it was customary for men to collect the blood and to gather together around it for a banquet in honor of the idols, to whom they held the blood to be most acceptable. Hence the Lord commanded the blood to be poured out and to be covered with earth (Lev. 17:13). For the same reason they were forbidden to eat animals that had been suffocated or strangled: because the blood of these animals would not be separated from the body: or because this form of death is very painful to the victim; and the Lord wished to withdraw them from cruelty even in regard to irrational animals, so as to be less inclined to be cruel to other men, through being used to be kind to beasts. They were forbidden to eat the fat: both because idolaters ate it in honor of their gods; and because it used to be burnt in honor of God; and, again, because blood and fat are not nutritious, which is the cause assigned by Rabbi Moses (Doct. Perplex. iii). The reason why they were forbidden to eat the sinews is given in Gn. 32:32, where it is stated that "the children of Israel. . . eat not the sinew. . . because he touched the sinew of" Jacob's "thing and it shrank."
The figurative reason for these things is that all these animals signified certain sins, in token of which those animals were prohibited. Hence Augustine says (Contra Faustum iv, 7): "If the swine and lamb be called in question, both are clean by nature, because all God's creatures are good: yet the lamb is clean, and the pig is unclean in a certain signification. Thus if you speak of a foolish, and of a wise man, each of these expressions is clean considered in the nature of the sound, letters and syllables of which it is composed: but in signification, the one is clean, the other unclean." The animal that chews the cud and has a divided hoof, is clean in signification. Because division of the hoof is a figure of the two Testaments: or of the Father and Son: or of the two natures in Christ: or of the distinction of good and evil. While chewing the cud signifies meditation on the Scriptures and a sound understanding thereof; and whoever lacks either of these is spiritually unclean. In like manner those fish that have scales and fins are clean in signification. Because fins signify the heavenly or contemplative life; while scales signify a life of trials, each of which is required for spiritual cleanness. Of birds certain kinds were forbidden. In the eagle which flies at a great height, pride is forbidden: in the griffon which is hostile to horses and men, cruelty of powerful men is prohibited. The osprey, which feeds on very small birds, signifies those who oppress the poor. The kite, which is full of cunning, denotes those who are fraudulent in their dealings. The vulture, which follows an army, expecting to feed on the carcases of the slain, signifies those who like others to die or to fight among themselves that they may gain thereby. Birds of the raven kind signify those who are blackened by their lusts; or those who lack kindly feelings, for the raven did not return when once it had been let loose from the ark. The ostrich which, though a bird, cannot fly, and is always on the ground, signifies those who fight God's cause, and at the same time are taken up with worldly business. The owl, which sees clearly at night, but cannot see in the daytime, denotes those who are clever in temporal affairs, but dull in spiritual matters. The gull, which flies both in the air and swims in the water, signifies those who are partial both to Circumcision and to Baptism: or else it denotes those who would fly by contemplation, yet dwell in the waters of sensual delights.