Tosafos asks according to Rashi. However, Firstly Rashi says that some say that R' Achai is speaking on erev shobbas and some during chol, and even if Tosafos only learnt that Rashi held that was during shobbas, Rashi finishes of saying that it is zman meuchar, (and in Parenthasis "Lekulam".)
Now while the second half of tosafos seems to be saying the same thing as Rashi, namely that in metzius the zman for T'vilas kohanim is miba'od yom while eating on erev shobbas is later, how does tosafos even begin? - If he is asking according to Rashi, Rashi says straight out that it is zman meuchar - hence and factually later than zman tvilas kohanim(what is the hava aminah?)
Moshe Rubin, Brooklyn, NY, USA
Dear Moshe,
Tosfos is not basing his question on Rashi's opinion that the time for eating is the latest of the times. All he says is that Rashi's opinion is that when Rebbi Achai mentions "the time of eating," he is referring to the time of eating on Erev Shabbos.
That Gemara asks that in different Beraisas Rebbi Meir mentions two times for saying Shema: the time of Tevilas Kohanim and the time that people eat on Erev Shabbos. Why is that a contradiction? Perhaps they occur at the same time? Rashi explains that one of those two times is later and the other is earlier, and that is what creates a contradiction in the opinion of Rebbi Meir.
Tosfos takes a different approach. Without determining exactly when each time is, Tosfos explains that the time of Tevilas Kohanim and the time that people eat on Erev Shabbos cannot occur at the same time. If they would occur at the same time, then in the second Beraisa Rebbi Meir's opinion (Tevilas Kohanim) will be the same as Rebbi Achai's opinion, since Rebbi Achai's opinion is that the time for Keri'as Shema is when people eat on Erev Shabbos (as Rashi says). Rebbi Meir cannot be saying the same time as Rebbi Achai since they appear in the same Beraisa as contending opinions. That is why Rebbi Meir's opinion in the second Beraisa (Tevilas Kohanim) contradicts what he says in the first Beraisa (when people eat on Erev Shabbos).
Afterwards Tosfos offers a second explanation for why there is a contradiction. He suggests that the time of Tevilas Kohanim must be earlier than the time that people eat.
All the best,
Reuven Weiner