Question: The opinion of Rebbi Meir has been switched - There (in the case of Chametz) he said that "ach'' comes to include (part of the 14th) and here (in the case in the first Perek of Beitzah where we learn from the word "ach'' that 'machshirei ochel nefesh' are prohibited on Yom Tov) he said that it comes to exclude?! (Pnei Moshe)

Answer (Rebbi Shmuel bar Avidoma): For Chametz it is also an exclusion, as one may not have Chametz on the 14th.

Rebbi Meir explains the pasuk (Devarim 16,3), "You shall not eat upon it Chametz'' applies to when eating the Pesach offering (but one could have Chametz during the afternoon of the 14th when one is only sacrificing it.) Rebbi Yehudah explains that the prohibition applies to when sacrificing it (during the afternoon of the 14th).

Rebbi Yehudah has a positive and a negative mitzvah both for eating Chametz and for destroying it -

The positive mitzvah against eating it is (Devarim 16,3),''Seven days you shall eat upon it matzah'', but not Chametz - any negative mitzvah that comes through a positive mitzvah is called a positive mitzvah.

The negative mitzvah against eating it is (ibid), "You shall not eat upon it Chametz''. (Rebbi Yehudah learns that if this pasuk prohibits Chametz at the time of sacrificing the Pesach offering, then all the more so Chametz will be prohibited during Pesach itself. However, according to Rebbi Meir, the pasuk refers to the time of eating the Pesach offering, but this would not apply to the rest of Pesach.)

The positive mitzvah of destroying Chametz is (Shemos 12,15), "...you must destroy leaven''.

The negative mitzvah for destroying Chametz is (Shemos 12,19), "For seven days leaven may not be found in your houses''.

(The Gemara taught in 1c): Rebbi Meir holds that the prohibition to eat Chametz after the 6th hour is Rabbinic.

Question: Only from the 7th hour [should it be prohibited](as Rebbi Meir holds that only the hour before sunset is a Torah prohibition, but the Rabbis extended the prohibition back to half way through the day, which begins at the 7th hour)-so why is the 6th hour prohibited?!

Answer: As a fence to protect the 7th hour.

Question: Do Chazal make a fence around a fence?

Answer: One will confuse the 6th hour with the 7th hour (consequently even the 6th was included in the original gezeirah).

Question: Rebbi Yehudah holds that the 5th hour is Rabbinic. If the 6th hour is prohibited as a fence, why would the 5th hour also be prohibited as a fence? Do we make a fence around a fence?

Answer: One will confuse the 5th hour with the 7th hour (consequently even the 5th hour was included in the original gezeirah).

Question: The opinion of Rebbi Yehudah has been switched - There (the Mishnah in Sanhedrin Ch.5 concerns a case of testimony - If one witness said that something took place at the 5th hour and one said it was at the 7th hour, the testimony is invalid as the sun is on different sides of the sky at those two times) he said that one would not confuse the 5th hour with the 7th hour!

Answer #1 (Rebbi Yosi) There, the matter is given over to Beis Din (witnesses) who act with alacrity [and they would not confuse the times]; here it is given over [even] to women who are sluggish [and they might err].

Answer #2 (Rebbi Yosi bar Bun): There refers to the beginning of the 5th hour and the end of the 7th hour (since there is such a great difference, one could not get confused); here refers to the end of the 5th hour and the beginning of the 7th (where the difference is minor).

Support (Beraisa): At the beginning of the 5th hour the sun is in the east and at the end of the hour it is moving towards the west. The sun only actually goes to the west at the end of the 7th hour (therefore these times are easily confused).