Commento su Levitico 14:57

להורת ביום הטמא — This may be translated: TO GIVE INSTRUCTION "CONCERNING" THE DAY OF ITS BEING UNCLEAN, i. e. which day renders it clean and which day renders it unclean (to point out that particular day on which, according to circumstances, a house or a garment or a person is to be declared clean or unclean).

זאת תורת הצרעת, one is not to add and impose more stringent rules also declaring similar looking skin eczemas as conferring ritual impurity. [the author interprets the word זאת, this, as restrictive, i.e. “this and none other.” Ed.] Granted that there are numerous other skin diseases which in some respects resemble the ones mentioned in the Torah, it is not in order to apply what the sages normally do to protect a person from inadvertently running afoul one of G’d’s commandments. In the domain of negai-im the rule postulated by Solomon in Proverbs 30,6 אל תוסף על דבריו, “do not add anything to G’d’s words” is strictly adhered to. One factor which indicates how wrong it would be to do this is that the Torah herself declares the “worst” phenomenon of skin eczema, i.e. the whole skin being affected totally, as one that leaves the afflicted party ritually pure. [Naturally, such a person, in the process of being cured will experience part of his skin being afflicted, at which point the priest will have to declare him as impure. Ed.] The apparent lack of logic in this part of the legislation was the cue for the sages not to impose “protective fences” around this legislation.

What day he is to declare [the house] pure. The ב of ביום טהרתו is like the ב of “ובחרושת אבן למלאות ובחרושת עץ (to cut stones for setting, to carve wood)” (Shemos 31:5), which means with regard to cutting stone. The same applies here as well, Scripture instructs with regard to the day of becoming impure and the day of becoming pure. [However, in Toras Kohanim the Rabbis taught: This teaches that in the day he declares impure and in the day he declares pure], that is, during the day but not at night, because we do not declare impure nor do we declare pure at night. [Rashi did not use this explanation because he already explained (v. 2): “on the day of his purification” — “This teaches that we do not proclaim him pure at night.” Also, from the verse above (13:14): “On the day that healthy flesh appears on him” the Rabbis derived (Moed Katan 8a): “This teaches that we do not proclaim him impure at night.”]