The Torah's prohibition to cook a kid goat in its mother's milk is understood by Chazal to prohibit the cooking of meat from cows, sheep, and goats, together with milk, even if it is not from the animal's mother. In shiur we explored the relationship between the simple sense of the verse and the way it is interpreted according to halakha. Why is there a gap between the literal and halakhic meaning of the verse, and in what way is the halakha of meat and milk shaped by the simple sense of the verse?
The first question, the reason for the gap between the literal and halakhic meaning of the verse, is not unique to this mitzvah. Perhaps the most well-known example of this is the verse that states that injury should be repaid "an eye for an eye," which is interpreted by the Rabbis to mean monetary compensation. If this is the meaning, though, why did the Torah articulate in this fashion? A number of Rishonim - including Ibn Ezra (Shemot 21:24) and Rambam (Laws of Bodily Injury, 1:3) - state that the Torah is expressing the fundamental principle. Monetary compensation can never truly rectify the taking of another's eye, it can never make the person whole. From the perspective of justice, what is truly deserving is not compensation, but an equal and opposite punishment. The Torah is saying that true justice demands "an eye for an eye." There is just one problem - such a punishment would be destructive societally. It would create a cycle of violence, and would desensitize individuals and society to violence. The way to take this principle and translate it into the real world, is to use monetary compensation as a substitute for taking the person's eye. The dialectic between the Written and Oral Torah is key - without the Oral Torah we would actually be gouging out eyes, but without the phrasing of "an eye for an eye," we would think that a person's suffering and loss can be reduced to, and compensated by, mere monetary payment. We would devalue the victim's loss, and lose sight of the heinousness of the perpetrator's crime. This dialectic informs us of both the principle at stake and of its real-world translation.
Taking this strategy and applying it to the case of a kid goat in its mother's milk, we can understand that the Torah is concerned, at the core, not with meat and milk per se, but specifically with a goat in its mother's milk. However, translating this into the real world, the categories get abstracted and expanded, so that all meat from domesticated animals represent a kid goat, and all milk represents mother's milk. This makes sense particularly if we focus on the reason given by Ramban for this prohibition. Ramban (Deut 14:1) explains that this prohibition parallels the Torah's prohibition of killing a cow and its calf on the same day. To cook a goat in its mother's milk, to be so insensitive to the taking of the milk that was created to nurture this goat, and instead us it to prepare the meat that came from its slaughter, would be an act that would desensitize us to suffering and breed within us the trait of cruelty. The goat in its mother's milk is the paradigm for this act of cruelty, but it can logically be extended to all meat - the product of an act of killing an animal - and all milk - a product that is created to nurture and support life. All meat represents the meat of kid goat, and all milk represents the mother's milk - the symbolism is similar, but less intensely felt. The goat and the mother's milk, thus, is the paradigm and the embodiment of the underlying principle; the practical application, however, translates and extends the prohibition to all meat and milk that is cooked together.
Another approach, however, is to state that the literal sense of the verse does not represent the principle or the paradigm, but merely a common example of a more general problem - diber ha-katuv bi-hoveh, Scriptures speaks of common cases (see Rambam, Moreh Nevukhim III:48). That is, the verse is not to be taken literally, or read in a narrow sense. In the case of "an eye for an eye," this approach would explain that this is just a stock phrase, and is not meant literally; in the case of the kid goat in the mother's milk the verse would be explained to be describing a common case (in those times), but the point is to prohibit all meat and milk cooked together.
These two approaches - one which treats the prohibition of meat and milk as a stand-in for or virtual kid goat in its mother's milk, and the other which understands the prohibition to primarily be that of milk and meat - have implications for the halakhic derivation and the halakhic parameters of the prohibition. Consider, first of all, the different ways the Rabbis derive that other meats are prohibited. In the Mekhilta (on Shemot 23:19) we find many derashot, uses of hermeneutical principles, to make this expansion. What stands out, however, is that one approach uses the three-time repetition of the prohibition to expand the verse - one verse tells use goat, one verse extends this to sheep, and the third verse extends this to cows. In contrast, Rabbi Akiva (also quoted in the mishna in Hullin 113), works in the opposite direction. He assumes the verse refers to all meat, and then uses the three-time repetition to exclude certain meats - one verse excludes non-kosher animals, one excludes wild animals (e.g., deer), and one excludes birds. An approach that begins with the assumption that the verse is only referring to a kid goat presumably maintains that perspective even in the end, and sees the other meats as stand-ins for the paradigm kid goat. In contrast, R. Akiva who begins with the assumption that the verse is referring to all meats, that is - that the verse is forbidding meat and milk - presumably ends with that perspective even in the end, and understands that although it is about meat and milk, not all meat is classic meat - only red meat (not fowl), only "normal" meat (not game), and only the type of meat that one would normally eat (and not non-kosher meat).
The difference of these approaches can also be discerned in practical halakha. Consider, first, the opinion of certain Tannaim that deer and other non-domesticated animals (chayot) are not included in the Biblical prohibition (this is also the way Rambam rules - although he seems to contradict himself, see Forbidden Foods 9:3 and Rebels 2:9). This makes total sense if the meat must represent a kid goat - such a stretch may be possible with other domesticated animals, but is much harder to make with non-domesticated species. On the other hand, if the primary prohibition is meat, then it is reasonable to assume (although, as mentioned above, not necessarily the case) that deer meat is included.
When we turn to the case of chicken and other fowl, the story gets more interesting. According to the Bavli, there is even a Tanna that is of the opinion that fowl are part of the Biblical prohibition (see Hullin 103b-104a, and 113a). Fowl in milk can in no way be a stand in for a kid goat in its mother's milk because it has no mother's milk! If it is Biblically prohibited according to this position it can only be because this Tanna understands that the primary Biblical prohibition is meat and milk, and hence all meat, even fowl, are included.
Now, we rule that fowl are permitted Biblically but prohibited Rabbinically. However, even this point is debated. The Talmud (Hullin 116a) states that this is true according to R. Akiva, but according to R. Yossi HaGlili, fowl are fully permitted -even from a Rabbinic perspective. The Talmud makes it clear (contra Rashi) that R. Yossi HaGlili's full permission of fowl and milk is linked to the way he derives the fowl exclusion of fowl from the verse:
Levi once visited the house of Joseph the fowler, and was served with a peacock's head cooked in milk and said nothing to them about it. When he came to Rabbi [Yehuda the Exilarch and related this]. Rabbi said to him: Why did you not place them under a ban [for violating the Rabbinic prohibition]? He replied: Because it was the place of R. Yehudah ben Betaira and I imagine that he must have expounded to them the verse like R. Yossi HaGlili who said: "A fowl is excluded since it has no mother's milk."
That is to say, because one emphasizes the exclusion of fowl on the basis that it is completely irrelevant to a prohibition about a goat in its mother's milk, it makes sense that even on a Rabbinic level we would not forbid it. It is as irrelevant to the prohibition as fish! On the other hand, if fowl are only excluded because they are not "classic" meat - which is the position of R. Akiva, as we have seen - then it makes sense to extend the prohibition Rabbinically to include all meats, which is exactly the position of R. Akiva.
Now, this assumes that the Rabbinic prohibition would only be a natural extension of the Biblical one. It is possible, however, to suggest that the Rabbis framed the prohibition on a Rabbinic level in a different way from how it was framed on a Biblical level. Biblically, the prohibition is a goat in its mother's milk, with the logical extension to sheep and cows, and even milk not from its mother. However, Rabbinically, the prohibition is framed as "meat and milk," and thus all meat and milk - even game meat, and even fowl meat - is prohibited. This is suggested by the first mishna in the 8th chapter of Hullin (103b), which states:
Every kind of meat is forbidden to be cooked in milk, excepting the meat of fish and of locusts
it is also forbidden to place upon the table [meat] with cheese, excepting the flesh of fish and of locusts
If a person vowed to abstain from meat, he may partake of the meat of fish and of locusts.
The mishna is concerned with defining what constitutes "meat," not only for meat and milk, but for other halakhic categories as well, and it is totally unconcerned with the question of what fits into the category of "a kid goat in its mother's milk." It is for this reason, I believe, that most halakhot regarding meat and milk do not distinguish between red meat and chicken, although one is Biblical and one is Rabbinic. Such is the case with the middle clause of the mishna which forbids even having chicken on the same table with milk (unless there is a divider). This raises the question why the Rabbis went so far - is this not a gezeirah li'gezairah - one Rabbinic safeguard on top of another one? The Gemara itself asks this question, and states that sometimes we can make such double gezeirot, although it does not explain why we did so here. The explanation, I believe, is that the Rabbis did not consider their prohibition of chicken as a gezeirah, a safeguard against the Biblical prohibition. Rather they considered what they had done as a reframing of the Biblical prohibition - from a "goat in mother's milk" to "meat and milk" - and from this perspective chicken and beef are equivalent. It should also be noted that one Rishon, the Issur vi'Heter (and possibly an opinion in Tosafot 104b, s.v. Of), actually rules that fowl is Biblically prohibited. According to this opinion, even from a Biblical perspective the prohibition is that of "meat and milk."
There is no question that as a lived practice, maybe in large part because we apply it to chicken, and this is the most common meat that we eat, we experience the prohibition to be that of "meat and milk." By understanding how the Rabbis, through interpretation and tradition, extended the literal "kid goat and mother's milk," to one degree, and then, through legislation, extended it even further, we can see how the shift from a literal kid goat and mother's milk, to a "virtual" kid goat and mother's milk, and finally, to our practice, of a simple but broad prohibition of all meat and milk.