레위기 15:28의 주석

ואם טהרה מזובה וספרה, But if she be cleansed from her issue, then she shall number, etc. We have to try to understand the use of the past tense for the word וספרה instead of the future tense, i.e. ותספור. Precisely when is the count to commence? If the count is to commence immediately, why does the Torah not write ותספור? If it is to occur only after she has become purified, the words "afterwards she shall be clean" do not make sense. We may have to explain this in light of what Torat Kohanim wrote on the meaning of the words טהרה מזובה. They interpret it as a cessation of the issue. The Torah tells us how we are to know that she may be considered cleansed from her issue, i.e. from the time the flow of blood stops. Her "purity" is then sufficient to permit her to begin the count of seven days (during which the flow must not recur). Rashi concurs with this interpretation in Megillah 8. He writes that she may begin counting without first having to immerse herself in a ritual bath.

ואחר תטהר, “and after that she is ritually clean.” Immersion of her whole body in forty measures of water in the ritual basin are what made her clean. Our sages in the Talmud tractate Eyruvin, folio 75 derive this from the words: אך במי נדה יתחטא, “but in the waters in which a menstruating woman bathes it will be cleansed.” [after waiting for the appropriate number of days without bleeding, Ed.] (Numbers 31,23). In verses 16-17 in our chapter, these steps to obtain ritual cleanliness are spelled out. Midrash Rabbah on Isaiah 8,6, describing the slow moving river Shiloah as moving לאט, points out that the numerical value of that word is 40; hence 40 saah of rainwater or well water is the minimum required for a ritual bath this has become the minimum amount stipulated by our sages as adequate for submersion.