(1) Adonai spoke to Moses and Aaron, saying to them: (2) Speak to the Israelite people thus: These are the creatures that you may eat from among all the land animals:
I maintain that the food which is forbidden by the Law is unwholesome. There is nothing among the forbidden kinds of food whose injurious character is doubted, except pork (Lev. 11:7), and fat (ibid. 7:23). But also in these cases the doubt is not justified. For pork contains more moisture than necessary [for human food], and too much of superfluous matter. The principal reason why the Law forbids swine's flesh is to be found in the circumstance that its habits and its food are very dirty and loathsome.... A saying of our Sages declares: "The mouth of a swine is as dirty as dung itself" (B. T. Ber. 25a).
The fat of the intestines makes us full, interrupts our digestion, and produces cold and thick blood; it is more fit for fuel [than for human food]. Blood (Lev. 17:12), and nebelah, i.e., the flesh of an animal that died of itself (Deut. 14:21), are indigestible, and injurious as food; Trefah, an animal in a diseased state (Exod. 22:30), is on the way of becoming a nebelah.
The characteristics given in the Law (Lev. xi., and Deut. xiv.) of the permitted animals, viz., chewing the cud and divided hoofs for cattle, and fins and scales for fish, are in themselves neither the cause of the permission when they are present, nor of the prohibition when they are absent; but merely signs by which the recommended species of animals can be discerned from those that are forbidden.
The reason for certain birds being forbidden as food is on account of their cruel nature. It is also possible that the reason for certain animals being forbidden is similar, since no animal that chews the cud and has a parted hoof is a beast of prey, while the rest all devour others. There has also been found a difference in nature between animals fit for food and those which are unfit, as the Sages have mentioned, namely that all milks of animals fit for food curdle, whereas all milks of those unfit for food do not coagulate and cannot ever be made into cheese. Thus they are physically different. It is possible to say on the basis of this difference in their natures that those animals unfit for food harm human beings who eat them.
(Responding to Nachmanides)…God forbid that I should believe such a thing! If that were the case then the Torah of the Lord would be no more than an insignificant and overly concise medical treatise. This is not the way of the Torah of the Lord or of its profound objectives. Besides, with our own eyes we see how the nations that consume the flesh of the pig, detestable things, the mouse as well as the other impure birds, land animals and fish, are all alive and well, strong and not at all feeble or frail…All of this is a clear indication that the Divine Torah did not come to heal the body or to promote physical health but rather to foster the health of the soul and to heal its afflictions. Therefore, the Torah forbade these foods because they have a deleterious effect on the pure and intelligent soul, breeding insensitivity in the human soul and corrupting its desires. This causes the formation of an evil nature that breeds a spirit of "tuma" and banishes the spirit of "tahara" and holiness, concerning which David implored: "Do not take Your spirit of holiness from me!" (Tehillim 51:13).
רַב אָמַר לֹא נִתְּנוּ הַמִּצְווֹת אֶלָּא לְצָרֵף בָּהֶן אֶת הַבְּרִיּוֹת, וְכִי מָה אִיכְפַּת לֵיהּ לְהַקָּדוֹשׁ בָּרוּךְ הוּא לְמִי שֶׁשּׁוֹחֵט מִן הַצַּוָּאר אוֹ מִי שֶׁשּׁוֹחֵט מִן הָעֹרֶף, הֱוֵי לֹא נִתְּנוּ הַמִּצְווֹת אֶלָּא לְצָרֵף בָּהֶם אֶת הַבְּרִיּוֹת.
Rav said: The mitzvot were only given so that man might be refined by them. Do you really think that The Holy One of Blessing cares if an animal is slaughtered by front or by the back of the neck? Therefore, mitzvot were only given to make humans better.
“You say you do not understand how these forbidden foods can be harmful. Even if you were never to understand and even if experience were never to teach you how the Divine spirit in man is linked with the body, would it not be enough for you that God, from whom alone in any case your power of these things derives, has refrained from giving you the right to partake of these foods! You should observe the commandments of the Torah and have regard for its laws, because they are at God’s behest, not because you think them correct. Even those commandments whose reason you believe you have understood, you should not fulfil because of your understanding, for then you would be listening only to yourself, whereas you should listen to God.”