The Bible dictates that "Thou shalt not seethe [= cook] a kid [= young goat]
in his mother's milk." To avoid any possibility of breaking that regulation,
the Jewish tradition ruled that it also applies to female goats, to mature
goats, and to the meat and milk of two completely unrelated goats. It is
also forbidden to eat the meat with fresh milk, and it applies to beef
and mutten as well (including mixing the milk and meat of two different
beasts). Finally, chicken, which are incapable of milk production, may not
be eaten along with any mammal's milk either.
We are fortunate that most mathematicians were not Jewish. Otherwise, it
would have been forbidden to divide by all numbers between -1 and 1.