The Mekhilta DeRabbi Yishmael presents a step in a larger legal argument about why meat cooked in milk is forbidden to eat. The passage uses a technique called refutation — countering an objection with a stronger parallel case.
An earlier argument had attempted to prove that meat and milk should be permitted to eat, based on the following logic: meat and milk, when cooked together, do not confer ritual impurity (tumah) when carried. Since they lack this particular severity, perhaps they also lack the severity of being forbidden to eat.
The Mekhilta demolishes this reasoning by pointing to fats and blood. The Torah explicitly forbids eating certain animal fats (chelev) and blood. Yet neither fats nor blood confer tumah when carried. They share the exact same characteristic that the earlier argument relied upon — no impurity by carrying — and yet they are unquestionably forbidden to eat.
The logic is airtight. If the absence of tumah-by-carrying were proof that something may be eaten, then fats and blood should be permitted. But they are not. Therefore, the absence of tumah-by-carrying proves nothing about whether something may be eaten. The two categories — ritual impurity and dietary prohibition — operate independently.
This refutation clears the way for the prohibition of meat and milk to stand on its own terms. Just because meat cooked in milk does not confer impurity by carrying does not mean it is permitted to eat. Fats and blood prove that eating prohibitions exist independently of impurity laws. The Mekhilta's argument is a masterclass in rabbinic logical reasoning, dismantling a flawed inference by producing a decisive counterexample.