The Gemara continues and says that if there are two half k'Zaysim on the
inside walls of the basin and they are not connected at all (by a thread of
dough), then they are both Batel. Ula adds that this leniency applies only
with regard to Chametz in the crevices of a basin. In a *house*, though,
two half k'Zaysim must be disposed of even if they are not connected. The
reason is because perhaps while sweeping the house, one will sweep the two
separate half k'Zaysim into one pile, combining them together to form a
full k'Zayis in one place.
Why does the Gemara need to add this reason to explain why two half
k'Zaysim of Chametz in a house must be disposed of? The Gemara just taught
that even if the two half k'Zaysim would be on the outside of the walls of
the *basin*, they would not be Batel to the basin and one would be Chayav
to get rid of them! Certainly, then, when they are on the floor of the
house, they are not Batel to anything, and thus each half of a k'Zayis by
itself must be destroyed!
(a) The MAHARAM CHALAVAH says that in a house, we might have thought that
small pieces of Chametz that are less than a k'Zayis do not have to be
disposed of, because of the Halachah of "Pirurin" (6b) which says that
small crumbs of Chametz are insignificant and Batel. In the kneading basin,
though, such small pieces of dough are still in good, usable condition, and
thus they are still significant and are not Batel. The Gemara concludes
that even in a house, small pieces of dough are not Batel because they
might combine together to make a k'Zayis when one sweeps the house. Since
they might get joined together and become significant, they are not Batel
even when they are separate. When the Gemara earlier (6b) said that small
crumbs are Batel, it was referring to dried crumbs that will never become
fit, or -- as the PISKEI RI'AZ explains -- it is referring to very small
crumbs which we do not fear will join together to become a k'Zayis. The
MAGEN AVRAHAM (OC 442:2) gives the same explanation.
Similarly, RABEINU DAVID answers that small pieces are normally Batel and
it is only when they are on a kneading basin that we say they are not Batel
because they might join together. That is, on the outside of the basin
itself they must be disposed of because of the concern that they might come
in contact with each other and form a k'Zayis amount of Chametz. The Gemara
concludes that a house is the same as the outside of the basin (and we do
not say that in the house they are Batel because they are so far apart from
each other).
(b) The MAGID MISHNAH (Hilchos Chametz u'Matzah 2:16) infers from the
Rambam that the Halachah that half of a k'Zayis on the floor of a house
must be disposed of applies only when the Chametz is not within the floor
boards. If the Chametz pieces are between the floor boards and serve to
fill up cracks in the floor, then they are Batel, just like the pieces of
dough on the inside of the kneading basin which serve a purpose for the
basin and thus are Batel.
(c) The RAMBAN (Milchamos) and RABEINU DAVID say that according to the RIF,this Gemara holds like Rav Huna who, earlier, did not reconcile the two
conflicting Beraisos by saying that one is referring to places in the
kneading basin which are "Makom Lishah" and the other is referring to
places which are not "Makom Lishah." Rather, Rav Huna explained that the
two Beraisos are arguing; one holds that a half k'Zayis on the inside of
the wall of the basin is Batel, while the other holds that it is not Batel
and must be destroyed. This Gemara (Rav Nachman) -- which discusses a half
k'Zayis on the floor of a house -- is in accordance with the Beraisa that
says that half of a k'Zayis is Batel even when it is not serving any other
purpose. Therefore, the Gemara has to teach that this is not so in a house,
and a half k'Zayis in a house is *not* Batel because it might be swept
together with another half k'Zayis to form a whole k'Zayis.
(d) TOSFOS (45a, DH Kan) has a different explanation of "Makom Lishah."
Tosfos says that when pieces of dough are *not* in "Makom Lishah," we are
more lenient, and not more stringent, and we say that it is Batel, because
the dough that one is kneading in the basin will not come into contact with
it and there is no concern that it will get mixed in with the Matzah and be
eaten. Therefore, a half k'Zayis on the floor of a house will certainly be
Batel (if not for the reason that they might be swept into one pile to form
a k'Zayis).
(e) RABEINI YECHIEL cited by TOSFOS RABEINU PERETZ suggests that small
pieces of Chametz are Batel not because of the Halachah of Pirurin (6b),
but because they are a half of a Shi'ur ("Chatzi Shi'ur"), and the Isur of
Bal Yera'eh does not apply to Chametz less than the minimum Shi'ur (a
k'Zayis). The TESHUVOS P'NEI YEHOSHUA (#15) also gives this answer.
However, if so, why should a half k'Zayis not be Batel when it is on the
rim or on the outside of the kneading basin? The P'nei Yehoshua concludes
that it must be that the basin being discussed has a lot of pieces of
Chametz around its outside, which all add up to a k'Zayis. Therefore, they
must be disposed of because the basin joins them all together. However, if
there were multiple pieces of a half k'Zayis of Chametz on the floor of a
house, it is not necessary to dispose of them because there is nothing to
join them together to be a k'Zayis. Therefore, it is necessary to give the
reason that one might sweep them together. This is also the way the KORBAN
NESANEL (3:2:100) learns the Gemara.
According to this explanation, we need to understand why one does not
transgress the Isur of Bal Yera'eh with half of a Shi'ur. We know that
Chatzi Shi'ur is Asur mid'Oraisa (as the Gemara (Yoma 74a) teaches with
regard to eating on Yom Kippur). Why is our Gemara assuming that Chatzi
Shi'ur is not Asur? The CHACHAM TZVI (#86) explains that Chatzi Shi'ur is
only Asur when it comes to Isurei Achilah -- forbidden food items. By
eating the item, no matter how small it is, one gives it importance and
therefore it is Asur.
Alternatively, the SHA'AGAS ARYEH (#81) explains, the Isur of
Chatzi Shi'ur does not apply to Bal Yera'eh for the following reason. Why
is it Asur to eat Chatzi Shi'ur of an Isur? The Gemara in Yoma says it is
Asur because the small amounts of food are "Chazi l'Itzterufi," they join
together to become a proper Shi'ur. This may mean that it is Asur to eat
one half-Shi'ur because if the person eats another half-Shi'ur, he will
have eaten an entire Shi'ur and will have transgressed the Isur d'Oraisa
*retroactively* by eating the first half-Shi'ur. Anything which can turn
into an Isur retroactively, is Asur. When it comes to Bal Yera'eh and Bal
Yimatzei, though, even if one had at the beginning of Pesach one half
k'Zayis in his possession, and then later during Pesach another half
k'Zayis came into his possession, he will only be in violation of Bal
Yera'eh and Bal Yimatzei at the point in time at which he has a full Shi'ur
in his house at once. The act of having the first half-Shi'ur alone can *never* be Asur,
because no matter what one buys afterwards, he has not transgressed anything
*until* he buys another half-Shi'ur (since at that point he has an entire
Shi'ur in his possession).
Many Acharonim reject the logic of this assertion, because Bal Yera'eh
should be Asur even with Chatzi Shi'ur for several reasons. (1) The MINCHAS
CHINUCH (11:12) says that even though that logic works to explain why there
is no Isur d'Oraisa of Chatzi Shi'ur for Bal Yera'eh, but mid'Rabanan, at
least, there should be an Isur of Chatzi Shi'ur. (2) The MAHARAM CHALAVAH
writes that we know that the prohibition of Bal Yera'eh, of not having
Chametz in one's possession, is in order to prevent one from eating Chametz
on Pesach. Consequently, even if the Isur of Chatzi Shi'ur applies only to
Achilah, the Shi'ur of Bal Yera'eh is based on the Shi'ur of Achilah, and
therefore Chatzi Shi'ur should be Asur mid'Oraisa for Bal Yera'eh just like
it is for Achilah (see also TAZ OC 442:5).