halacha - Why is milk with meat not kosher?

I've recently read (again) the verse "לא תבשל גדי בחלב אמו", meaning, "Do not cook a young goat in its mother’s milk".

How did anyone get from that, to "Don't eat meat with milk at all"?

Answer

See here for more.

Basically, if you look carefully in Biblical Hebrew, g'di actually means "a young animal" -- usually if you didn't specify it meant a goat, but it could be a generic term for any young. Thus elsewhere it might specify g'di izim -- "a young goat."

So that gives us "don't cook a young animal in its mother's milk."

Why the thing about "mother's"? Hebrew lesson once again, the language is written without vowels. "Chalav" is milk; "chelev" is fat. So by adding in the phrase "mother's" we know it's talking about cooking it in milk, not fat.

We believe that an Oral Tradition was given along with the Bible as we know it, which meant that this verse was intended as:

Don't cook the meat of any ruminant animal in the milk of any ruminant animal.

The verse appears thrice in the Torah, giving us: don't cook it, don't eat it if it was cooked, and don't sell it if it was cooked.

In fact you'd only violate the direct Torah prohibition if it was cooked with milk; if you took a cold hamburger and soaked it in milk for an hour, that isn't in the prohibition. Centuries later, the rabbis of the Talmud added a prohibition of their own, knowing that if you okayed people to eat such a hamburger, it's likely they'll reheat it - thus getting to a Torah prohibition.

That pertains as the legal definition. As for the ethical message, Rabbi Samson Raphael Hirsch suggests that meat is taking, and milk is giving. Rabbi Abraham Isaac Kook suggests going back to the simple reading of the verse -- once you kill an animal, its mother's milk has no use. To then take that milk and use it to make the meat extra-tasty is too blatant of a disrespect for animal life.