Sanhedrin

When R. Isaac came,  he taught as we have learnt [in our Mishnah]: R. Judah said, he incurs guilt only on account of her maternal relationship to him.  Now why is this? — Abaye said: Scripture saith, The nakedness of thy father, or the nakedness of thy mother, shalt thou not uncover, she is thy mother.  [This teaches:] You must punish him for maternal incest, but not for incest with his father's wife. If so, what of the verse, The nakedness of thy father's wife shalt thou not uncover: It is thy father's nakedness?  Does it not imply, you may penalise him for incest with his father's wife, but not for maternal incest? In that case, if she is both his mother and his father's wife, one verse implies the exclusion of maternal incest [as the incriminating offence] — and the other excludes incest with his father's wife [as punishable].  Now if he is punished for incest with his mother, even when not his father's wife, and with his father's wife, though not his mother-shall we say that when she is both his mother and his father's wife, he incurs no penalty at all? A further difficulty is this: Do not the Rabbis admit the existence of this verse, 'she is thy mother'?  But they interpret it as teaching the law deduced by R. Shisha, the son of R. Iddi;  in that case, R. Judah must also utilise it for the same purpose.  But R. Aha the son of Ika said thus: The Writ sayeth: [she is thy mother: thou shalt not uncover] her nakedness.  This teaches: You may penalise him for one degree of 'nakedness', but not for two degrees,  If so, what of the verse: Thou shalt not uncover the nakedness of thy daughter-in-law: She is thy son's wife: thou shalt not uncover her nakedness?  Does this too teach: You may penalise him for one degree of 'nakedness', but not for two? But we have learnt: HE WHO COMMITS INCEST WITH HIS DAUGHTER-IN-LAW INCURS A PENALTY IN RESPECT OF HER BOTH AS HIS DAUGHTER-IN-LAW AND AS A MARRIED WOMAN. (HE IS GUILTY IN RESPECT OF HER BOTH DURING HIS SON'S LIFETIME AND AFTER HIS DEATH); and R. Judah does not dispute this! But since she is but one person, though forbidden in a double capacity, the Writ saith, 'her nakedness' [singular]: here too then, [in the case of one's mother who is also the father's wife] since she is one person, even if she were doubly forbidden, the Writ saith: 'her nakedness'.  — But Raba answered thus: R. Judah maintains that the nakedness of thy father [thou shalt not uncover], means thy father's wife, deducing this by a gezerah shawah,  and it applies to her whether she is his mother or not; whence do we know then that one's mother who is not his father's wife is likewise forbidden? — From the verse, the nakedness of thy mother shalt thou not uncover. [Hence the phrase,] 'she is thy mother' teaches that he is guilty only on account of her maternal relationship, but not because she is his father's wife.