Sefer HaChinuch ספר החינוך

To not cook meat in milk: That we not cook animal meat in animal milk, as it is stated (Exodus 23:19), “you shall not cook a kid in its mother’s milk.” And the explanation came (Chullin 113a) that it is not specifically a kid, but rather all meat of an animal is implied — as the expression, “kid” is an expression that only [applies to] animal meat. And the verse [chose] the expression, “kid,” since meat is a soft thing, like a kid. And how is it that you learn like this? From that which you find in several places in the Torah where it is written, “kid,” and it was necessary [for the Torah] to explain, “a goat kid.” Behold, you have learned [from this] that in a place where it is stated only, “kid,” it is not specifically a goat kid, but rather all animal meat like it is implied.

It is from the roots of this commandment, according to what appears, that it is similar to the matter that we wrote about the commandment of the witch (Sefer HaChinukh 62), that there are things in the world the mixture of which are forbidden to us, for the reason of the matter that we said there. And it is possible that the reason for the prohibition of the mixture of meat and milk through the process of cooking is from this foundation. And there is somewhat of a proof to this, since the prohibition comes to us with the action of mixing, even though we do not eat it; such that we see from this that its prohibition is not because of the damage [caused by] its eating at all. Rather [it is] that we do not do the action of that mixing, for the sake of the distancing of that matter that we said. And we have also been warned in another place, that if we maybe make the mixture, we should not eat it and not derive benefit from it, to distance the matter [further]. And he is lashed even if he ate it without deriving benefit (enjoyment) from it at all (Pesachim 25a), which is not the case with all the other forbidden foods. And all of this teaches that the foundation of its reason is because of the mixture, and like the matter that we said about magic. This is said out of duress, and we still need the knowledge of the mystic. And Rambam, may his memory be blessed, said (Guide for the Perplexed 3:48) that there are some idolaters that worship through the act of mixing meat and milk. And that is why the Torah distanced that mixture. “But none of this is worthwhile for me.”

We will write the laws of the commandment in brief, with God’s help — as is our custom — in the commandment of the prohibition of eating and benefit, in the Order of Ki Tissa (Sefer HaChinukh 113).

And [it] is practiced in every place and at all times by males and females. And one who transgresses it and cooks meat with milk is lashed — and even though he does not eat it.