Dear Rabbi Kornfeld,
Thank you so much for your response to my question regarding the to'ah of nidah 29b. I thought of a possible answer, and I would be interested to know if you think it has any validity. The gemara says that we must be going according to Rabbi Shimon who says that even though the woman should be permitted on her 7th clean day after she immerses because miktsas hayom k'kulo, still she is forbidden just in case she would later see blood. But tosafos ask why not say it is even according to the Rabbis (not R.Shimon) who would certainly say she is not permitted on the 7th clean day because they don't say miktsas hayom k'kulo there.
So maybe the answer to that question (according to Levi) is that since Rabbi Shimon usually (not in a case of yoledes) says that there is a GEZAYRA against the woman on the 7th clean day, so "lo ploog" and that gezayra still applies to the yoledes even though there is really no reason for her to be forbidden. In other words, the 21st day is forbidden so the woman will not assume that we EVER say she is permitted on the 7th clean day after only part of the day. The rabbis who disagree with Rabbi Shimon hold that the 7th day is actually forbidden without the gezayra, so they don't say "lo ploog", and according to them she would be permitted on the 21st day after tevillah. That is why the gemara says we are going according to the opinion of Rabbi Shimon.
Thanks again for your time.
Judah Koolyk
"Lo Plug" is an excellent answer, Judah! I still didn't find anyone who discusses your question, but Lo Plug appears to be what must be said to answer the Sugya according to Levi. (It still sounds a bit strange to say Lo Plug as far as Tashmish on day #7, when, on the other, hand Levi allows her to count days that she actually saw blood as clean days and there is no Lo Plug -- since no Gezeirah is involved. In fact, if Rebbi Shimon prohibits Tashmish mid'Oraisa, it may be even easier to explain the Sugya; he prohibits Tashmish during Yemei Tohar due to a Gezeirah that it will be confused with normal days. Although no such Gezeirah was made with regard to blood during Yemei Tohar causing her to recount, nevertheless in this case, that's because nobody will confuse the issue since blood obviously causes a recount of clean days. Tashmish without blood, though, is easily to be confused. But you could be correct, nonetheless, even if it's a Gezeirah mid'Rabanan that prohibits Tashmish according to Rebbi Shimon.)
As for what you wrote about the Rabanan who disagree with Rebbi Shimon, I'm not sure that I agree on that point. There are two approaches in the Rishonim as to what the Rabanan hold, and none of the approaches contend that they don't say Miktzas Hayom k'Kulo on the 7th day of a Zavah (nor does such an opinion appear to be recorded in Shas elsewhere).
(1) According to Tosfos (29b, 67b) the Rabanan (Beis Hillel, Mishnah 72a) hold that Tashmish is Asur after Tevilah on day #7 because of the Safek that she might see blood later, but the Tevilah is permitted. Rebbi Shimon prohibits Tevilah as well, on Daf 67b (except in the more lenient case of the To'ah, where he accedes to the Rabanan, while the Rabanan permit even Tashmish.) According to this view, Rebbi Shimon never made a Gezeirah against Tashmish per se, so your answer of Lo Plug remains invalid according to Tosfos' approach.
(2) According to the Rashba and Rishonim, the Rabanan indeed permit even Tashmish, since they are not afraid that the woman will see blood suddenly after six clean days. If so, it is obvious why our Sugya cannot conform to the Rabanan's opinion.
Perhaps you mean that if we make a compromise, and learn that the Rabanan prohibit Tashmish mid'Oraisa (like Tosfos) due to the Safek, while Rebbi Shimon only prohibits Tashmish mid'Rabanan, and not Tevilah at all(like the other Rishonim), then you can suggest your answer as to why the Sugya mentions only Rebbi Shimon. However, I'm not sure that such a compromise can be drawn, because if Tashmish is prohibited mid'0raisa according to the Rabanan, there is absolutely no source for asserting the Rebbi Shimon permits is mid'Oraisa. That is why Tosfos understood that Rebbi Shimon argues only with respect to Tevilah, and not Tashmish.
In any case, we appreciate your sharing your sharp thoughts on this matter with the Kollel. Be well,
-Mordecai