QUESTION: Zeiri and Rav disagree about whether "Eshes Avi Imo" -- the wife of the father of one's mother -- has a "Hefsek" or not. Is that relationship prohibited for only that generation (the wife of the father of one's mother) or is it prohibited for preceding generations as well (such as the wife of the father of one's grandmother)? Zeiri maintains that "Eshes Avi Imo" is not prohibited in any other generation. Rav maintains that the prohibition extends to earlier generations.
Rav explains that although there is no Ervah d'Oraisa involved in the first generation (since his mother cannot have a wife), nevertheless the Chachamim decreed that "Eshes Avi Imo" is prohibited for all prior generations because of the existing prohibition of "Eshes Avi Aviv," the wife of the father of one's father, which does have an Ervah d'Oraisa in the first generation ("Eshes Aviv," the wife of one's father). Since the wife of his mother's father and the wife of his father's father are easily confused, the Chachamim gave them the same status and prohibited both.
Zeiri disagrees and says that they are not easily confused. A person spends far more time with his father's family than with his mother's family, and thus he will not confuse the two.
According to Zeiri's view that one does not confuse his maternal relatives with his paternal relatives, why did the Chachamim prohibit one's father's mother (his paternal grandmother) without a "Hefsek" for all previous generations? RASHI (DH v'Eshes Achi ha'Av) explains that although there is no woman in the first generation that corresponds to the relation of one's father's mother, nevertheless the Rabanan prohibited this relation for all generations because she is confused with the grandmother on the maternal side, one's mother's mother. Everyone, including Zeiri, agrees with this. Why, then, is Zeiri not concerned that one will confuse the maternal and paternal sides in the case of "Eshes Avi Imo," the wife of one's mother's father?
ANSWER: RASHI (DH Hasam) alludes to the answer to this question. Rashi writes that because a person spends far more time with his father's family he considers the relatives on that side to be "more closely related" to him than those on the maternal side. Thus, there is more reason to prohibit a relative on one's paternal side.
This is the reason why, according to Zeiri, the Chachamim did not prohibit the maternal side even though the paternal side is prohibited. Accordingly, the wife of one's mother's father is not prohibited. The Gemara earlier discusses the opposite situation -- prohibiting the paternal side because the maternal side is prohibited. If one's mother's mother is prohibited, then the Chachamim should also prohibit one's father's mother because the paternal side is always treated more stringently due to the higher degree of closeness one feels with his relatives on the paternal side.
According to Rashi, this is the intent of the Gemara: the fact that one spends more time with his paternal relatives is more reason to prohibit them to him. This approach supports the words of the RAMBAM and IBN EZRA (quoted by the RAMBAN to Vayikra 18:6) that the prohibition of Arayos is intended to prohibit a man from marrying the women with whom he associates and interacts most often so that he not act in the way of animals who mate with all of the females around him. (M. Kornfeld)