Deuteronomy is literally translated as “second law”, just as is משנה תורה. In being a repetition, the book is of great interest as an interpretation of the preceding books of the Torah.
Its selection of laws to repeat and to add apparently shows different priorities to other books that have been noted by commentators since the time of its writing.
An act of intepretation which interests me is the placement of the commandment “thou shalt not cook a kid in its mother’s milk” (Deut. 14:21).
From earlier references (Exod. 23:19, Exod. 34:26), it is unclear that this law has anything to do with food. There, the statement is one in a list of laws which are not greatly connected one to another; its immediate context is pilgrimage, offerings and first fruits. (The second context is almost identical to the first, only occurring after Moses’ receipt of a second set of tablets.)
The Deuteronomy passage focusses on forbidden foods: only some animals are appropriate to be eaten; and carcasses of animals which have not been slaughtered are not to be. And then it states “do not cook a kid in its mother’s milk”, just as it did in Exodus.
(Curiously, in all cases, the statement ends a section.)
From the Exodus context alone, one would not assume that this commandment had any real impact on diet. It may only relate to sacrificial ritual: perhaps it was a pagan or idolatrous practice; Seforno suggests that it was assumed to help one’s crops or flocks. It could have particular relation to pilgrimage: Ramban and Ibn Ezra suggests this was a time that young livestock would be present with lactating mothers; Rashbam suggests that the festive season was a time for meat. Or it may be a mere ethical matter: Rashbam and Ibn Ezra compare the law to not killing an animal and its offspring in one day, and to sending off the mother bird before taking her eggs.
Unsurprisingly, many commentators pick up on the law’s food context in Deuteronomy. Certainly, the rabbinic understanding of the law as a prohibition of eating or cooking meat and milk together is only really afforded validity by it’s citation here. Perhaps this also explains the famous statement that this law appears three times “once to prohibit eating, once to prohibit benefit, and once to prohibit cooking” (BT Hulin 113b, 115b; Rashi on Exod. 23:19); the Halakhic midrash could only take one mention to refer to food, because in Exodus this doesn’t seem to be the point.
In a way, Jewish approaches to the bible often treat it as a commentary to itself. The midrash constantly connects multiple passages in ways that may have not been obvious at first, and hence treats the relationship between two apparently disparate texts.
The interpretative act apparent in the text of Deuteronomy — as a repetition of the law — allows us to see these connections within the bible, without first applying the easily-distorted lens of midrash and later commentaries.