I remember learning that the reason you don't eat milk with beef is because you shouldn't cook a calf in its mother's milk. However, when you are dealing with two different species, this same logic doesn't seem to hold. Is there another explanation for why this isn't kosher?
The Torah's prohibition of cooking a young animal in mother's milk, as stated above, applies to mammal's meat in mammal's milk. Rabbi Samson Raphael Hirsch writes that meat is about taking, and milk is about giving.
As birds don't have "mother's milk", there is no Biblical prohibition on chicken-cooked-in-milk. However, the rabbis of the Talmud prohibited it, for fear of mistakes. This distinction is an important one when understanding Judaism, and is stressed by Maimonides.
Well, m'doraita it is applied to all meat and all milk from animals similar to goats (domesticated, kosher animals like cows and sheep, but would exclude foul, fish, and non-domesticated like deer and no-nkosher like pigs). The passuk just mentions it that way because that was a common way of doing it. Chazal darshen three issurim from the three times it appears in Chumash
- The issur to cook meat and milk together
- The issur to eat meat and milk cooked together
- The issur to benefit from milk and meat cooked together (Such as selling meat cooked with milk.)
Chazal also made additional laws on top of the basic laws of the Torah to prevent people from making mistakes. Rabbi Yose haGalili permitted milk and chicken to be cooked together, because chicken is a bird and the law of the Torah is about four-legged animals. Rabbi Akiva disagreed and throught people could get confused between different kinds of meat. Halakha is like Rabbi Akiva.
I'm not Jewish, but I always assumed the 'reason' for all Jewish food laws is 'because god says so!' If you're looking for any other rationalisation, you're missing the point. There's nothing uniquely unhealthy about pork or lobster, but god says don't eat them, so don't! Yeah, there's symbolism of life and death with milk and meat... But that doesn't stop Jews (or others) eating eggs, or killing and eating young animals. Basically, if you expect god to justify his commandments before you follow them, you're not respecting his will.
The Rambam in the Mishneh Torah, Mamrim, Chapter 2, Halacha 9 discusses the issue of when it is considered adding to the Torah by the Rabbis. In this Halacha he describes the concerns they may have had when the Rabbis prohibited cooking fowl and milk together. He also clarifies what the Torah command is and what the Rabbinic command is.
What is implied? The Torah states Exodus 23:19: "Do not cook a kid in its mother's milk." According to the Oral Tradition, we learned that the Torah forbade both the cooking and eating of milk and meat, whether the meat of a domesticated animal or the meat of a wild beast. The meat of fowl, by contrast, is permitted to be cooked in milk according to Scriptural Law. Now if a court will come and permit partaking of the meat of a wild animal cooked in milk, it is detracting from the Torah. And if it forbids the meat of fowl cooked in milk saying that this is included in "the kid" forbidden by the Scriptural Law, it is adding to the Torah.
If, however, the court says: "The meat of fowl cooked in milk is permitted according to Scriptural Law. We, however, are prohibiting it and publicizing the prohibition as a decree, lest the matter lead to a detriment and people say: 'Eating the meat of fowl cooked in milk is permitted, because it is not explicitly forbidden by the Torah. Similarly, the meat of a wild animal cooked in milk is permitted, because it is also not explicitly forbidden.' "And another may come and say: 'Even the meat of a domesticated animal cooked in milk is permitted with the exception of a goat.' And another will come and say: 'Even the meat of a goat is permitted when cooked in the milk of a cow or a sheep. For the verse mentions only "its mother," i.e., an animal from the same species.' And still another will come and say: 'Even the meat of a goat is permitted when cooked in goat's milk as long the milk is not from the kid's mother, for the verse says: "its mother."' For these reasons, we will forbid all meat cooked in milk, even meat from fowl."
כֵּיצַד. הֲרֵי כָּתוּב בַּתּוֹרָה (שמות כג יט) "לֹא תְבַשֵּׁל גְּדִי בַּחֲלֵב אִמּוֹ". מִפִּי הַשְּׁמוּעָה לָמְדוּ שֶׁזֶּה הַכָּתוּב אָסַר לְבַשֵּׁל ולֶאֱכל בָּשָׂר בְּחָלָב. בֵּין בְּשַׂר בְּהֵמָה בֵּין בְּשַׂר חַיָּה. אֲבָל בְּשַׂר הָעוֹף מֻתָּר בְּחָלָב מִן הַתּוֹרָה. אִם יָבוֹא בֵּית דִּין וְיַתִּיר בְּשַׂר חַיָּה בְּחָלָב הֲרֵי זֶה גּוֹרֵעַ. וְאִם יֶאֱסֹר בְּשַׂר הָעוֹף וְיֹאמַר שֶׁהוּא בִּכְלַל הַגְּדִי וְהוּא אָסוּר מִן הַתּוֹרָה הֲרֵי זֶה מוֹסִיף. אֲבָל אִם אָמַר בְּשַׂר הָעוֹף מֻתָּר מִן הַתּוֹרָה וְאָנוּ נֶאֱסֹר אוֹתוֹ וְנוֹדִיעַ לָעָם שֶׁהוּא גְּזֵרָה שֶׁלֹּא יָבוֹא מִן הַדָּבָר חוֹבָה וְיֹאמְרוּ הָעוֹף מֻתָּר מִפְּנֵי שֶׁלֹּא נִתְפָּרֵשׁ כָּךְ הַחַיָּה מֻתֶּרֶת שֶׁהֲרֵי לֹא נִתְפָּרְשָׁה. וְיָבוֹא אַחֵר לוֹמַר אַף בְּשַׂר בְּהֵמָה מֻתֶּרֶת חוּץ מִן הָעֵז. וְיָבוֹא אַחֵר לוֹמַר אַף בְּשַׂר הָעֵז מֻתָּר בַּחֲלֵב פָּרָה אוֹ הַכִּבְשָׂה שֶׁלֹּא נֶאֱמַר אֶלָּא אִמּוֹ שֶׁהִיא מִינוֹ. וְיָבוֹא אַחֵר לוֹמַר אַף בַּחֲלֵב הָעֵז שֶׁאֵינָהּ אִמּוֹ מֻתָּר שֶׁלֹּא נֶאֱמַר אֶלָּא אִמּוֹ. לְפִיכָךְ נֶאֱסֹר כָּל בָּשָׂר בְּחָלָב אֲפִלּוּ בְּשַׂר עוֹף.
From the Rambam it would seem that the Rabbis were concerned that misconceptions about the law would arise due to the ambiguity of the prohibition described in the Torah. One might think if bird and milk can be cooked together, then perhaps it is only a goat and its milk that would be prohibited, but a goat and cow's milk would be fine. False deductions would be made were the prohibition not extended to cooking birds and milk together. The Rabbi's job is to protect the Torah level commandment. They did so by extending the prohibition to help clarify the legal categories involved and show the extent of the Torah level prohibition.