The Torah in Parashat Re’eh (Debarim 14:11) commands, “Kol Sipor Tehora Tochelu” – “You shall eat every pure species of bird.” The Sefer Ha’hinuch, following the Rambam, understands this verse as introducing the command to determine the kosher status of birds. As discussed in the context of an earlier Misva (153), the Torah lists twenty-four species of birds which are forbidden for consumption, and all other birds in the world may be eaten. In theory, then, one who knows the identity of these twenty-four species can eat any species of bird which he has determined to not belong to any of these species. The Sages taught that a bird can be confirmed kosher it has all of the following four properties: it does not attack other creatures with its claws; it has an extra toe; it features a crop; and it has a gizzard that can be peeled.
The Ramban disagrees with the Rambam, and does not consider it a Misva to inspect birds to determine their kosher status. In his view, this verse adds a Misvat Aseh (affirmative command) to the Misvat Lo Ta’aseh (prohibition) against eating non-kosher species of birds, such that one who partakes of a such a bird transgresses both a Misvat Lo Ta’aseh and a Misvat Aseh.
After listing the twenty-four non-kosher species of birds, the Torah commands that one may not eat “Sheretz Ha’of” (14:19), referring to small flying insects, such as bees and hornets.
The reason behind these commands is the same as the reason underlying the restrictions on the consumption of other creatures, which the Sefer Ha’hinuch explained in a separate context, namely, that the species which the Torah designated as forbidden damage the body. The body serves the vital function of containing the soul, and thus if we compromise the body’s wellbeing, this adversely affects the soul. Partaking of non-kosher species impairs the body, and thus, by extension, it impairs the soul.
This idea is expressed in a letter which the Rambam had his student, Ibn Tibbon, write to a certain community that had contacted him asking for guidance. The people in this town were struggling with certain matters involving Jewish faith, as the people were finding it difficult to accept and embrace certain principles of Emuna (such as the belief in the future resurrection). The Rambam told his student to respond, and the student wrote a letter advising the people to check to ensure that the food they were eating was permissible. He explained that when a person eats, the digestive system processes the food, expelling the waste and distilling what it is kept in the body. The finest components of the food are sent to the brain, so it can function at optimum level. Therefore, if one consumes non-kosher food, the brain receives low-quality nourishment, and this compromises its functionality, impeding its ability to properly perceive and understand. Therefore, Ibn Tibbon urged the townspeople to determine whether perhaps they were eating non-kosher food, and this was hampering their ability to accept the core principles of Jewish faith.
For this same reason, when Abraham Abinu set out to teach and guide people toward monotheism, he offered hospitality, inviting guests and giving them food. At first glance, we might think that this was simply a tactic, a way to bring people under his influence. But it has been explained that Abraham wanted them to ingest kosher food in order that their minds would then be receptive to the concepts of Emuna which he sought to teach them. Likewise, the Zohar comments that Beneh Yisrael needed to eat Masa at the time of Yesiat Misrayim in order for them to be able to achieve Emuna. The Zohar draws an analogy to a doctor who instructs a parent not to give the ill child normal food until he finishes the prescribed medication, because only once the malady is cured will the child’s body have the ability to properly process food. By the same token, Beneh Yisrael were spiritually ill, deficient in Emuna, and they required “medication” to cure them of this malady. Masa, the Zohar teaches, is “the food of faith,” with the power to bring spiritual healing, and Beneh Yisrael could not eat Hametz until they first ate Masa for seven days to cure themselves of their spiritual illness and achieve firm belief in Hashem.
The Rama (Rav Moshe Isserles, Cracow, 1530-1572), in Yoreh De’a, writes that a woman who eats non-kosher food should not nurse an infant, because the child would then ingest milk produced from non-kosher food, which would have a deleterious spiritual effect. The commentaries (Shach and Taz) explain that the Rama refers to a case where a woman needs to eat certain non-kosher food for medical reasons due to a life-threatening condition. This is a situation where the mother is allowed – and even required – to eat non-kosher food, and nevertheless, the milk she produces could contaminate the child because of the non-kosher food she ingests. If this is the effect of non-kosher food consumed permissibly, then certainly, non-kosher which one eats in violation of the Torah causes severe spiritual harm.
These commands, of course, apply in all times and places, and are binding upon both men and women. If a person eats a Ke’zayit from a forbidden species, or an entire forbidden creature (“Birya”) regardless of its size, is liable to Malkut.