The Mekhilta raises a fascinating question about the relationship between laws that existed before the giving of the Torah at Sinai and those that were introduced at Sinai itself. The specific case involves two well-known dietary prohibitions: the thigh sinew and the mixture of meat and milk.
The prohibition against eating the thigh sinew — the gid hanasheh, the sciatic nerve — predates the Torah. It originates in (Genesis 32:33), after Jacob wrestled with the angel and was struck on his hip. Because of that primordial encounter, the children of Israel do not eat the thigh sinew to this day. This prohibition was already binding before Moses ever ascended Sinai.
The Mekhilta considers whether we might use the thigh sinew as a template for other prohibitions. Since the thigh sinew was forbidden before the Torah was given, and it is indeed forbidden to be eaten, one might argue that any food prohibition should follow the same logic. But the text immediately challenges this reasoning.
Meat cooked in milk was not forbidden before the giving of the Torah. This prohibition appears only in the Torah itself, repeated three times in (Exodus 23:19), (Exodus 34:26), and (Deuteronomy 14:21). Since meat and milk was not prohibited in the pre-Sinai era, one might argue that it should not carry the same weight as the thigh sinew. The Mekhilta uses this distinction to explore how different historical origins of commandments affect their legal classification and application.