Is this based on bas echas or collel?
yehuda, st louis, usa
Isur Mosif is an independent category of 'Isur Chal al Isur'. It is not a branch of Isur bas Achas or Isur Kollel.
In fact, our Sugya seems to consider it stronger than Isur Kollel. What is the difference between the two?
'Isur Kollel' is where the new Isur adds more 'pieces' on to the person to whom the one piece is already Asur; whereas 'Isur Mosif' is where the existing Isur becomes forbidden to more people, without adding any Isur to the person on whom it is already forbidden.
For example: If Reuven marries Rachel, then Le'ah, Rachel's sister, becomes forbidden to him because of Achos Ishto. When Shimon then marries Le'ah, since she now becomes forbidden to Reuven (and Shimon)'s brothers (because she is Eishes Achiv), she will also be forbidden to Reuven because of Eishes Achiv (even though she is already forbidden to him because of Achos Ishto), when Shimon dies. That is an Isur Mosif.
Whereas if Shimon had married Le'ah first, Le'ah would have been forbidden to Reuven because of Eishes Achiv. When he (Reuven) then marries Rachel, since he now becomes forbidden to marry all Rachel and Leah's sisters (because of Achos Ishto), he will also be forbidden to marry Le'ah (because of Achos Ishto) after Shimon's death. That is an Isur Kollel.
Be'Virchas Kol Tuv
Eliezer Chrysler
Two (unrelated) questions on Issur Kollel/Mosif:
1) In your second scenario, where Shimon marries Leah first, and then Reuvain marries Rachel, Leah becomes assur to Reuven as achos Eishto because of Issur Kollel.
What if there were no other sisters (other than Leah and Rachel) would this still be an Issur Kollel? In other words, does issur Kollel actually need to be Kollel other things or does it just need potential?
2) When an Eishes Ish becomes a niddah we say this is an issur Mosif, because she is now becoming assur to her husband also. If we look at it thru the Niddah's perspective it should be an issur Kollel. Yesterday she was permitted to her husband, now that she becomes a niddah her husband is assur . Since the issur niddah is chal on her husband it should be chal on all men. Why don't we look at it as an issur Kollel?
(I heard an approach to this question, but I'm curious thear your response)
Kol Tuv
Henach Grumet
Far Rockaway NY
1. The Lashon the Gemara uses regarding Isur Kolel (and Isur Mosif) is "Migu ..." - "since he becomes Asur to all the sisters, he also becomes Asur to the sister-in-law as well". This certainly implies that the Isur Kolel must actually take effect, and not just potentially. Otherwise, the Migu would make no sense.
The Yosef Da'as cites the Rambam (Shegagos 4:3) and Rishonim who, based on a Gemara in Kerisus (14b), say this explicitly with regard to Isur Mosif.
2. The Torah forbids a man to have relations with a Nidah ("ve'el Ishah be'Nidas Tum'asah Lo Sikrav ... " [Vayikra 18:19]). In that case, the man is the subject, and the woman, the object of Isur, in the way that I explained. The fact that the Torah elsewhere includes the woman in all Isurei Arayos will not change that.
be'Virchas Kol Tuv
Eliezer Chrysler.