Not All the Milk is Dairy
Here is a riddle: what comes from milk, must be classified as milk in order to be Kosher, but is not milk and may be cooked with meat?
The answer is - whey, the liquid residue from cheese production. And you thought rabbis lead a boring life?
Let's explain this.
Milk, like any other component of a cow, giraffe, sheep, deer or goat, is prohibited if taken from a living beast. Accordingly, there is no such thing as Kosher milk unless it is collected from a cow that has been Shechted.
The Gemara explains however, that the Torah provides a special exemption for milk. Without this special exemption, milk from a living cow would not be Kosher.
When milk is processed to manufacture cheese, it is split into two components: curds and whey. Only the curds are identified as milk. The whey however, is not milk. By Torah Law it may be cooked with meat.
RaMBaM applies this re-definition rigorously. Since it is not milk, it does not enjoy the special exemption conferred upon milk, and it is prohibited as a product taken from a living beast. He concludes that since whey is no longer milk, it may be cooked with meat. If it was milk, we would not be permitted to even cook it with meat and meat cooked with whey would be banned from all benefit. We may not use it as pet food for example.
However, since whey is not milk it may be cooked with meat. At the same time though, whey is not Kosher, so the meat we are permitted to cook with it is not Kosher. It may be used though, as pet food.
The Rosh [Chulin Ch8] quotes Rabbenu Simcha who, with a stroke of genius, suggests that the whey can simultaneously be both milk and not milk.
It is milk with respect to enjoying the special exemption that ensures it remains Kosher. However, it is not milk with regard to cooking it with meat.
He explains that the Torah prohibits cooking meat in its mother's milk. This phrase is not just a poetic term, but a Halachic qualification. The only milk that may not be cooked with meat, is milk that emerges from a mother. Once it is altered and is no longer the same as it was when first emerging from the cow, it is transformed into a product that may be cooked with meat.
I think the greatest Chiddush of this discussion is the genius of Rabbenu Simcha. Where does he draw the substance of his Chiddush? The answer is, there is no Gemara that supports this interpretation, it is only the power of the human mind. LaMa Li KeRa? SeVaRa Hu. No verse in the Torah is required to teach us that which is self evident.
I think Rabbenu Simcha is a testimony to HKBH's plan, that we use our minds to see what seems reasonable and have the confidence to understand that this is the Torah that HKBH wants us to follow.