OPINIONS: The Mishnah (end of 14a) teaches that it is possible to be liable for six Chata'os for one act of relations, when the woman is forbidden to him because of multiple Isurim. The Gemara asks how multiple Isurim can take effect on one woman, if the Halachah is "Ein Isur Chal Al Isur" (a second prohibition cannot take effect on something that is already forbidden). The Gemara explains that when the additional Isur is an Isur Kolel or an Isur Mosif, a second Isur can take effect even when there is a pre-existing Isur. Similarly, when two Isurim occur b'Vas Achas (simultaneously), both take effect.

The Gemara continues and describes how one woman can be forbidden to the Bo'el with multiple Isurim (see Chart #2): The Bo'el first has relations with his mother and they have a daughter. The Isurim of Bito and Achoso (one's daughter and one's sister) take effect simultaneously. The daughter then marries the Bo'el's brother, causing the Isur of Eshes Ach to take effect on all of the Bo'el's brothers, as well as on the Bo'el (an Isur Mosif).

Finally, after the daughter is widowed or divorced, she marries the brother of the Bo'el's father. Since the Isur of Eshes Achi Aviv takes effect on the other brothers, it takes effect on the Bo'el as well.

There are several ways to understand the third step of the case that the Gemara describes.

(a) RASHI (DH Nis'as) explains that when the daughter marries the brother of the Bo'el's father, she becomes forbidden to the other brothers of his father because of the Isur of Eshes Ach. Consequently, she also becomes forbidden to the Bo'el because of the Isur of Eshes Achi Aviv. Although the Bo'el's Isur is not the same as the Isur that takes effect on the father's brothers, it still is considered an Isur Mosif.

(b) Rashi quotes a different Girsa, which says that she becomes forbidden to the Bo'el "since an Isur has been added with regard to the sons of his brother" (in contrast to our Girsa, which says "the brothers of his father"). That is, when she marries the brother of the Bo'el's father, she becomes forbidden to that brother's sons. Since an Isur of Eshes Aviv is added to her with regard to the Bo'el's cousins (his uncle's sons), she becomes forbidden to the Bo'el as well with the new Isur of Eshes Achi Aviv.

Again, the additional Isur that takes effect for the Bo'el is not the same Isur that takes effect for the others, but nevertheless it is consider an Isur Mosif. There is an element of Chidush in this point because one might have assumed that in order to have the status of an Isur Mosif, the same Isur must be added to the other people.

(c) The ARUCH LA'NER gives a third explanation. He asserts that the proper Girsa should be, "since an Isur has been added with regard to the rest of the sons of the father's brother." That is, the woman married the brother of the Bo'el's father. The Bo'el's father had a second brother as well, who has sons. These sons (the nephews of the woman's present husband) become forbidden to the woman because of Eshes Achi Aviv. Just as the Isur of Eshes Achi Aviv takes effect on the sons of the second brother of the Bo'el's father, the same Isur -- Eshes Achi Aviv -- takes effect on the Bo'el! (Mordechai Zvi Dicker)

QUESTION: In the Mishnah, Rebbi Yosi states that if the Zaken (the father of the Bo'el) transgressed and married the daughter of the daughter of the Bo'el, the Bo'el is Chayav for Eshes Av as well. (See Chart #2, scenario 2:8.) The Gemara explains that Rebbi Yosi maintains that "Ein Isur Chal Al Isur," one Isur cannot take effect when a pre-existing Isur is already in effect, unless it is an Isur Mosif, and, in this case, it indeed is an Isur Mosif. At the time that the woman becomes the wife of his father (Eshes Aviv), she becomes prohibited to others as well. By becoming prohibited to the son of the grandfather, she becomes prohibited to the Bo'el as well through Isur Mosif.

This is difficult to understand. If the "grandfather" that the Gemara mentions is the "Zaken" mentioned in the Mishnah -- that is, the father of the Bo'el -- then all of his sons are already forbidden to this woman, even before she marries the Zaken, since it is evident from the Mishnah that this woman had already been married to the brother of the Zaken (and thus she is the Eshes Achi Aviv, the wife of his father's brother) of the Bo'el and his brothers! (See Chart #2, scenario 2:4.) Accordingly, all of the Zaken's sons are already forbidden because of "Eshes Achi Aviv"! What Isur Mosif is there? (SHITAH MEKUBETZES #10)

(a) RASHI (DH Kegon, and as cited by the Shitah Mekubetzes) explains that the Gemara means merely that the sons of the Zaken indeed become prohibited to this woman because of "Eshes Av." However, according to Rashi, what Isur Mosif is there? It seems that Rashi explains the Gemara this way because he maintains that an added Isur is not considered an Isur Mosif unless the added Isur takes effect not only on someone else, but on the subject of the first Isur as well (see Background, Girsa #6).

However, this approach seems to be contradicted by the case of Kalaso (his daughter-in-law), for the Isur of Eshes Achiv makes her prohibited only to someone else, and yet this Isur is still considered an Isur Mosif to make her prohibited to the Bo'el because of "Kalaso" (see Chart #2, scenario 2:2), as the Shitah Mekubetzes (#10) points out. Thus, Rashi's explanation requires further elucidation.

(b) The RAMBAM's text of the Gemara reads, "When there is a minor (Katan) son to the grandfather," and he explains (in Perush ha'Mishnayos) that the son of the Zaken is still a Katan. When he matures (with the growth of two hairs, the sign of adulthood), the Isur of Eshes Av and the Isur of Eshes Achi Aviv take effect upon him b'Vas Achas, at one time.

(c) However, the Rambam, in the later edition of Perush ha'Mishnayos (as cited by the TOSFOS YOM TOV) and in the Yad ha'Chazakah (Hilchos Shegagos 4:2), retracts his explanation. He writes that this entire passage in the Gemara is an addition of the Rabanan Savora'i, and it is an error, because the Gemara's question is not really a question. The simple understanding of why the wife of the Zaken is prohibited through Isur Mosif is that the father of the Bo'el had brothers (and not sons). By being married to the Zaken, the woman becomes prohibited to the brothers because of Eshes Ach. Therefore, it is called an Isur Mosif with regard to the Bo'el, who will be Chayav because of Eshes Aviv.

Our text of the Gemara is the text of the Shitah Mekubetzes (#10) -- "when there is a son to the grandfather," but he explains that the "grandfather" here is the grandfather of the Bo'el -- that is, the father of the Zaken mentioned in the Mishnah. If he has a son, then he is the Achi Aviv (the brother of the father) of the Bo'el, and this brother becomes prohibited to the woman because of the Isur of Eshes Ach when the Zaken (his brother) marries her, making an Isur Mosif. This is exactly like the Rambam's understanding mentioned above (except that the Shitah Mekubetzes learns this way in our text, without making any emendations).