Midrash su Levitico 13:23

4) (Vayikra 13:20) "its appearance is lower": This tells me (that it is tamei) only if its appearance is lower. Whence do I derive for inclusion (an appearance that is) level (with the skin) or higher? From (Vayikra 13:21) "and it is not lower than the skin," (which indicates that "lower than the skin" is not a categorical requirement for tumah). "it has blossomed in the boil": but not in the skin of the flesh (outside of the boil). I might think that (to become tamei) it does not spread in the skin of the flesh, but it does spread in the skin of the burn (near the boil); it is, therefore, written (Vayikra 13:23) "And if in its place the bahereth stands — (if) it has not spread." In its place (in the boil) it spreads, (to become tamei); it does not spread in the skin of the flesh or in the skin of the burn.

5) (Vayikra 13:18) ("And flesh, if there be in it, in its skin, a boil, and it be healed") "a boil": (If only "a boil" were written), I would think that a "rebellious" (active one) were intended; it is, therefore, written "and it be healed." If "and it be healed," I would think that it must have formed a scab; it is, therefore, written "boil." How is this to be reconciled? It has healed and not healed. (i.e., it has formed a peel but not a scab.) And thus is it stated below (Vayikra 13:23) "it is the peel of the boil," forming a membrane similar to a garlic peel.

9) (Vayikra 13:23) ("And if the bahereth remains in its place — if it has not spread — then it is the peel of the boil, and the Cohein shall declare him tahor.") (lit.) "And if under it the bahereth stands — if it has not spread." It spreads (for purposes of conferring tumah) to the place under it (i.e., the boil). It does not spread (for such purposes) to the skin of the flesh (around it) or to the skin of a burn (alongside it). "the peel": Its place must be compact and distinct. "the Cohein shall declare him tahor": He declares tahor what is a certainty and not what is in doubt. How so? Two came to a Cohein (for inspection); one, with a bahereth the size of a garis; the other, (with a bahereth) the size of a sela. At the end of the week, the size in both is a sela and more. Both are tamei. Even if they recede to a sela in both, they are both tamei — until they recede to the size of a garis. In this connection it is written "and the Cohein shall declare him tahor." He declares tahor what is a certainty and not what is in doubt.