The Torah in Parashat Mishpatim (23:19) introduces for the first time the prohibition against “cooking a kid in its mother’s milk,” which has been understood as forbidding basar be-chalav – cooking or eating meat with milk, or deriving benefit from such a mixture.
The Gemara in Masekhet Bekhorot (6b) considers viewing this verse as the Biblical source for the permissibility of drinking the milk of a kosher animal. Intuitively, we would have assumed that since the Torah forbids partaking of that which originates from something forbidden for consumption (like the milk of non-kosher animals, or eggs of a non-kosher bird), even milk from a kosher animal should be forbidden, since kosher animals are forbidden for consumption until they are slaughtered. Since a cow’s milk, for example, is taken from a live cow, which is forbidden for consumption (as it is still alive), the milk, intuitively, should be forbidden. As tradition allows drinking the milk drawn from a live kosher animal, there must be some Biblical source which establishes that such milk is permissible. The Gemara briefly considers the possibility that the source of this law is the Torah’s prohibition against consuming milk with meat. The clear implication is that milk on its own, without meat, is permissible, thus seemingly establishing the permissibility of drinking milk.
However, the Gemara immediately dismisses this line of reasoning. It notes that even if milk were forbidden for consumption, the Torah would still have needed to introduce the prohibition of basar be-chalav to teach the additional unique features of this prohibition. As mentioned, the Torah forbids not only eating milk with meat, but also deriving other forms of benefit from such a mixture (like feeding it to one’s animal), and the very act of cooking milk and meat together. Therefore, the fact that the Torah needed to introduce this prohibition does not necessarily prove that milk by itself is permissible for consumption.
A number of later writers questioned the Gemara’s initial line of reasoning, for various reasons. One question that was raised is that even if milk itself were forbidden, the Torah would have still needed to introduce the law of basar be-chalav to forbid eating meat that had absorbed milk. Rashi, commenting to Masekhet Chulin (98b), controversially posits that the principle of ta’am ke-ikar, which forbids eating food that had absorbed the taste of prohibited food, does not apply on the level of Torah. According to Rashi, Torah law forbids the consumption of foods designated as prohibited, but does not forbid eating food that was cooked with such foods and thus absorbed their flavor. This prohibition, in Rashi’s view, was enacted by Chazal. According to Rashi, then, even if milk itself were forbidden, it would have been necessary for the Torah to forbid eating meat that had been cooked with milk, which would have otherwise been permissible for consumption. The question thus arises as to how the Gemara initially suggested that the basar be-chalav prohibition proves the permissibility of milk.
Among those who addressed this question was the Noda Bi-yehuda. In one of his published responsa (Mahadura Tinyana, Y.D. 36), he suggests that Rashi stated his opinion only in explaining the view held by some Amoraim. Rashi concedes, however, that others view ta’am ke-ikar as a Biblical provision, and it is thus possible that the Gemara here works within this latter position. (In an earlier responsum (33), the Noda Bi-yehuda suggests a different answer.)
Another answer was suggested by the Imrei Emet, the third Rebbe of Ger (cited in Ish Ha-eshkolot, p. 192). Significantly, Rashi viewed ta’am ke-ikar as a rabbinic law with regard to all prohibitions except basar be-chalav. The absorbed taste of milk in a piece of meat is forbidden on the level of Torah law even according to Rashi, because the Torah forbade eating a mixture of meat and milk “derekh bishul” – the way they would normally be cooked together, meaning, meat boiled in milk. The Imrei Emet thus suggested that the challenge to the Gemara’s initial proposal noted above is included in the Gemara’s ultimate refutation of this proposal. As we saw, the Gemara refuted this argument by noting that the Torah needed to introduce the basar be-chalav prohibition to add that meat and milk may not even be cooked together. This aspect of the prohibition results in the notion of “derekh bishul,” which, as explained, yields the application of ta’am ke-ikar to meat cooked with milk, such that it is forbidden by virtue of the flavor of milk which it had absorbed. And thus, indeed, this challenge is in fact part of the Gemara’s reasoning in rejecting its initial suggestion.