Resources for Kesubos 23
1. The גמרא says in the first אוקימתא of the ברייתא that if one witness says a woman is מקודשת and another witness says she isn’t then she shouldn’t get married לכתחילה but if she did she can stay married. The גמרא is not clear as to what the woman herself says. The רמב"ם in הלכות אישות פּרק ט הל׳ א writes something fascinating: he says that in the case above, if the lady says she isn’t מקודשת then she can’t get married לכתחילה. However, if it is just her against another עד where she says she isn’t מקודשת and he says she is, she can get married even לכתחילה. The ראב"ד there is משיג that this can’t possibly be—why would it be that if she is by herself she get married לכתחילה but if she has another עד agreeing with her then she cant!? Therefore, he disagrees and says that if the woman is agreeing with an עד that she was not married then she can get married לכתחילה, but if it is just her against the one עד then she cant get married לכתחילה. According to the ראב"ד the גמרא that says that in a case where one עד says she is married and the other says she is not must be talking about a case where she doesn’t know herself if she is married such as a case of ספק קרוב לו ספק קרוב לה. What would the רמב"ם answer to the ראב"ד’s question? The מגיד משנה answers that the רמב"ם understood that if she has an עד אחד backing her then she is more empowered to lie (מעיזה ומעיזה). Therefore, if she is on her own she is fully believed but if she is supported by an עד she should not get married לכתחילה.
2. תוספות in ד"ה תרוייהו asks why we don’t allow her to remarry לכתחילה when it is one עד against another עד because of the principle of חזקה and this lady has a חזקת פּנויה? תוספות answers that it must be a case where it was ספק קרוב לו ספק קרוב לה so the חזקה is broken. The אבני מילואים in סימן כ"ז ס"ק י"ח brings the מהרי"ט who understands תוספות to be saying that the חזקה is broken מדאורייתא but it hasn’t reached the level of being איתחזק איסורא either. Therefore, she can’t get married לכתחילה but if she did she doesn’t need to get divorced. The אבני מילואים doesn’t understand this at all since the notion of לכתחילה and בדיעבד is never said by a דאורייתא and is only by דינים דרבנן. He therefore says that תוספות holds it is just a חומרא דרבנן to not get married. The fascinating יסוד of the אבני מילואים that there are no בדיעבדs by a דין דאורייתא is mentioned explicitly in תוספות in גיטין דף ג׳ ע"ב ד"ה וכי. Interestingly, תוספות in פּסחים דף י"א ע"א ד"ה קוצרין seems to disagree and says there are many cases of בדיעבד by מצוות דאורייתא as we know by קדשים where we say שינה עליו הכתוב לפּוסלו. רבינו דוד there says קדשים is different. (I personally always wondered why מצוה בגדול ליבם wasn’t a clear example of לכתחילה by a דאורייתא.)
3. The גמרא says that בשבויה הקילו. תוספות on דף ל"ו in ד"ה ואלו says that this proves that a שבויה is only a ספק דרבנן or חז"ל would not have been able to be lenient However, theרמב"ם in הלכות איסורי ביאה פּרק י"ח הל׳ י"ז says the only reason חז"ל were able to be lenient with a שבויה is because all ספיקות are really מותר מדאורייתא. This is consistent with the רמב"ם ‘s opinion in all of ש"ס that ספק דאורייתא לחומרא is only מדרבנן.