The Torah divides animals into four categories: 1) domesticated and wild animals, that is, mammals; 2) fish; 3) birds; and 4) creeping creatures, including reptiles, aquatic creatures, flying insects, and the kosher locusts. In each of these categories there are kosher species and non-kosher species, as will be explained in this chapter.
For domesticated animals, wild animals, fish, and creeping creatures, the Torah provides identifying criteria that distinguish kosher species from non-kosher ones. For birds, it uses a different method: it lists the non-kosher species, thereby teaching that all others are kosher.
Several great Rishonim note a characteristic trait of kosher species: they eat grass and have a gentle temperament, whereas non-kosher species are predatory and aggressive. Since the food a person eats affects his soul, the Torah commanded us not to eat species inclined toward cruelty (Ramban, Rabbeinu Baḥya, and Abarbanel to Vayikra 11; Akeidat Yitzḥak, Shemini, Gate 60).
Some wrote that kosher species are healthy to eat, whereas non-kosher species are unhealthy (Guide for the Perplexed 3:48). Many objected to mentioning this reason, arguing that it is improper to diminish the Torah by turning it into a book of remedies. Moreover, we do not find that gentiles are physically less healthy than Jews. Rather, the mitzvot are meant to sanctify Israel and refine their souls; when a Jew eats forbidden foods, he defiles his soul. Therefore, the Torah calls the species that may not be eaten temei’im — impure (Akeidat Yitzḥak, Shemini, Gate 60; Abarbanel to Vayikra 11).
The Sages likewise taught: “Sin dulls a person’s heart, as it says: ‘Do not become impure through them, lest you become impure through them’ (Vayikra 11:43). Do not read venitmetem — ‘lest you become impure’ — but venitamtem — ‘lest you become dulled’” (Yoma 39a). All sins, and especially eating forbidden foods, dull the heart, making it unable to receive sacred spiritual matters. In this respect, forbidden foods are more severe than other prohibitions, for the forbidden food enters the body, becomes integrated into it, and damages it (Ramban, Rabbeinu Baḥya, Akeidat Yitzḥak, Abarbanel, and Malbim to Vayikra 11).
