If so, why should Rebbi Shimon *ever* rule about a Safek Tum'ah in Reshus
ha'Yachid that it is Tamei? In a normal Safek Tum'ah there are no grounds
to suspect that the object did indeed become Tamei!
(a) RASHI (DH v'Iba'is) and TOSFOS (DH u'Sheneihem) assert that Rebbi
Shimon indeed rejects the familiar rule of "Safek Tum'ah b'Reshus ha'Yachid
Tamei," and instead rules that every such Safek is *Safek* Tamei.
(b) The RAMBAN, however, disagrees. Normally, Rebbi Shimon rules that
"Safek Tum'ah b'Reshus ha'Yachid" is Tamei for *certain*, as we are
accustomed to hearing. Therefore, if there is a question as to whether an
action which can make an object Tamei did or did not take place, we rule
that the object in question is Tamei for certain. In the case of the
Mikvah, however, we only consider the person to be Safek Tamei, since we
know for *certain* that the Mikvah was once *measured* and found to contain
40 Se'ah. Rebbi Shimon does not compare to Sotah a case where there is a
doubt as to when or whether an object changed its status (and we see no
specific action taking place that could cause the change in status), since
there is a Chazakah d'Me'ikara in such a case which allows us to assume
that the status did not change, or that it changed later.
But why didn't Rebbi Shimon just say so, then? Why did the Gemara mention
that Sotah is different from Mikvah because there are "Raglayim l'Davar" to
consider her Tamei, instead of saying that Mikvah is different because it
has a Chazakah that it had 40 Se'ah?
The Ramban explains that even when there *is* a Chazakah, we still judge
the object (in our case, the Sotah) to be Tamei when there is Raglayim
l'Davar. Since there is no Raglayim l'Davar to make the person Tamei in the
case of the Mikvah that was found lacking, Rebbi Shimon does not rule that
it is Tamei for certain but that it is Safek Tamei.