Midrash su Levitico 11:47

7) (Vayikra 11:47): "to distinguish between the unclean and the clean": It should say "between cow and ass." And are they (their differences) not already explained (in the Torah)? What, then, is the intent of "between the unclean and the clean"? Between what is unclean to you (through perfect recognition) and clean to you: between shechitah of the greater part of the windpipe (after shechitah of the gullet, in which case it is kasher), and shechitah of half of it (after shechitah of the gullet, in which case it is neveilah). And what constitutes that difference? A full hairsbreadth.

8) "between the animal which may be eaten and the animal which may not be eaten": It should say "between the deer and the arod (a wild ass)." And are they not already explained? If so, what is the intent of the above? Between a treifah (an animal with a defect), which is kasher (for eating) and a treifah that is unfit. Others say: "between the animal that may not be eaten": This is an exhortation (against violation of a negative commandment) vis-à-vis (eating) an (unclean) animal.