Gemorrah says that by shevuyah we are makil because is a mere possibility (chashashah bealmah, rashi). However, earlier in the masechta, on 13b, the gemmorah indicated that once it was confirmed that a woman was a shevuyah, then it is as good as edim (lezuh yesh edim, lezuh ein edim) that she was violated. How do these fit together?
michael rubin, bklyn, new york
The key to answer your question lies in the fact that the Chumra of Shevuyah is mid'Rabanan.
Consequently, the Chachamim generally assume that a captured woman was raped. And that Rabinical assumption they gave the power of 'Anan Sahadi' (we are witnesses).
At the same time however, if there are no witnesses that she was captured, the "Peh she'Asar" overrides that assumption, and we apply the Sevara "bi'Shevuyah Heikilu", to be lenient even more than we would in other similar cases of "Peh she'Asar", as the Gemara explains here.
And it is by the same token that we are lenient in other cases of Shevuyah (later on Daf 27b and elsewhere), wherever there is the slightest Sevara to do so.
In a nutshell, when there is no leniency to rely on, we place the Shevuyah on a Chazakah of having had relations with her captor. When there is, then we consider her Tahor, on the basis of "b'Shevuyah Heikilu".
Be'Virchas Kol Tuv.
Eliezer Chrysler