Yevamot

but that a [Rabbinical] prohibition is 'nevertheless involved!  — The law, in fact, is that even a [Rabbinical] prohibition is not involved; only, because it was desired to state in the final clause, 'but are guilty [of a punishable offence]', it was stated in the first clause also, 'they are not guilty [of a punishable offence]'. Raba stated: With reference to the Rabbinical statement that [legally] an Egyptian has no father,  it must not be imagined that this is due to [the Egyptians'] excessive indulgence in carnal gratification, owing to which it is not known [who the father was], but that if this were known  it is to be taken into consideration;  but [the fact is] that even if this is known it is not taken into consideration. For, surely, in respect of twin brothers, who originated in one drop that divided itself into two, it was nevertheless stated in the final clause,  that they 'neither participate in halizah nor perform levirate marriage'.  Thus it may be inferred that the All Merciful declared their children to be legally fatherless,  for [so indeed it is also] written, Whose flesh is as the flesh of asses, and whose issue is like the issue of horses. Come and hear what R. Jose related: It once happened with the proselyte Niphates  that he married the wife of his [deceased]  maternal brother,  and when the case was submitted to the Sages their verdict was that the law of matrimony does not apply to a proselyte. But then, should a proselyte betroth a woman, would also the betrothal be invalid? — Say then rather: The prohibition of a brother's wife does not apply to a proselyte. Now does not [this refer to the case] where his brother  had married her while he was a proselyte!  — No; where he married her while he was still an idolater.  But if [betrothal took place] while he was still an idolater, what [need is there] to state it?  — It might have been assumed that [in the case of a brother's betrothal] while he is still an idolater a preventive measure should be enacted lest [erroneous conclusions be drawn in the case] where he is a proselyte, hence we were taught [that no such measure was enacted]. Come and hear what Ben Yasyan  related: When I went to the coastal towns  I came across a certain proselyte who had married the wife of his maternal brother. 'Who, my son', I said to him, 'permitted you [this marriage]?' 'Behold', he replied. 'the woman and her seven children;  on this bench sat R. Akiba when he made two statements: "A proselyte may marry the wife of his maternal brother", and he also stated, "And the word of the Lord came unto Jonah the second time, saying,  only a second time did the Shechinah speak to him; a third time the Shechinah did not speak to him."'  At any rate, it was stated here that 'a proselyte may marry the wife of his maternal brother'. Does not [this refer to a case] where his brother married her while he was a proselyte! — No; where he married her while he was still an idolater.  What [need then was there] to state [such an obvious law]? — It might have been assumed that [in the case of a brother's betrothal] while he is still an idolater a preventive measure should be enacted lest [erroneous conclusions be drawn in the case] where he is a proselyte. hence we were taught [that no such measure was enacted]. Is he,  however, believed? Surely R. Abba stated in the name of R. Huna in the name of Rab: Wherever a scholar gives directions  on a point of law and such a point comes up for a practical decision, he is obeyed if he made the statement  before the event;  but if it was not so made, he is not obeyed!  — If you wish I might say: The incident occurred after he made his statement. If you prefer, I might say: Because he stated, 'Behold the woman and her seven children'.  And if you prefer I might say: Here it is different  because with it he related another incident. The Master said, 'And the word of the Lord came to Jonah a second time, saying,  only a second time did the Shechinah speak unto him, a third time the Shechinah did not speak to him'. But surely it is written in Scripture, He restored the border of Israel from the entrance of Hamath unto the sea of the Arabah, according to the word of the Lord, which He spoke by the hand of His servant Jonah the son of Amittai, the prophet!  — Rabina replied: He  referred to the affairs of Nineveh. R. Nahman b. Isaac replied, It is this that was meant:  According to the word of the Lord … which He spoke by the hand of his servant, the prophet,  as his intention towards Nineveh was turned from evil to good, so was his intention towards Israel, in the days of Jeroboam the son of Joash, turned from evil to good. Come and hear: A proselyte who was born in holiness  but was not conceived in holiness  has [legally] maternal consanguinity but no paternal consanguinity. For instance:  If he married his maternal sister,  he must divorce her;  if his paternal one, he may retain her.  His father's maternal sister he must divorce;