Yevamot

how  could an Egyptian of the second generation ever attain purity!  But is not this possible when he transgressed and did marry one?  — Scripture  would not have written of a case of 'when'.  Behold the case of the bastard which is one of 'when'  and yet Scripture did write it!  — It wrote of a 'when' [leading] to a prohibition;  it would not have written of a 'when' [if it led] to permissibility.  Behold the case of the man who remarried his divorced wife,  which involves a 'when' [leading] to a permitted act  and yet did Scripture write of it! — In that case it was written mainly for the purpose of the original prohibition. Our Rabbis taught: If the expression of sons  was used, why was also that of generations  used; and if that of generations was used, why also that of sons?  If the expression of 'sons' had been used and not that of 'generations'  it might have been assumed that only the first and second son is forbidden but that the third  is permitted, the expression of 'generations'  was, therefore, used. And had the expression of 'generations' only been used and not that of sons,  it might have been assumed that the precept was given only to those who stood at Mount Sinai,  the expression of sons' was therefore used.  Unto them,  Count from them.  Unto them.  Be guided by the status of the ineligible among them. It was necessary [for Scripture] to write Unto them,  and it was also necessary for it to write, That are born.  For had the All Merciful written only. 'That are born', it might have been presumed that the counting must begin from their children,  hence did the All Merciful write 'Unto them'.  And had the All Merciful written only 'Unto them',  it might have been presumed that, where a pregnant Egyptian woman became a proselyte, she and her child are regarded as one generation. hence did the All Merciful write. 'That are born'. It was, furthermore, necessary to write Unto them  in this case,  and Unto him  in respect of the bastard. For had the All Merciful used the expression here only,  [the restriction  might have been assumed to apply to this case only], because the child descended from a tainted origin.  but not to a bastard, since he is descended from an untainted origin.  And had the All Merciful written the expression in respect of the bastard, [the restriction  might have been presumed to apply to him only]. because he is for all time unfit to enter into the assembly, but not in this case.  [Both texts were, therefore,] required. Rabbah b. Bar Hana stated in the name of R. Johanan: If an Egyptian of the second generation married an Egyptian woman of the first generation, her son is [regarded as belonging to the] third generation. From this it is obvious that he  is of the opinion that the child is ascribed to him. R. Joseph raised an objection: R. Tarfon said, 'Bastards may attain to purity. How? If a bastard married a female slave, their child is a slave. When, however, he  is emancipated he becomes a free man'.  This  clearly proves that the child is ascribed to her! — There it is different, because Scripture said, The wife and her children shall be her master's. Raba raised an objection: R. Judah related, 'Menjamin, an Egyptian proselyte. was one of my colleagues among the disciples of R. Akiba, and he once told me: I am an Egyptian of the first generation and married an Egyptian wife of the first generation; and I shall arrange for my son to marry an Egyptian wife of the second generation in order that my grandson shall be eligible to enter the congregation'.  Now, if it could be assumed that the child is ascribed to his father, [he could have married a wife] even of the first generation!  — The fact is that  R. Johanan said to the Tanna:  Read, '[a woman of the] first generation'. When R. Dimi came  he stated in the name of R. Johanan: If an Egyptian of the second generation married an Egyptian wife of the first generation. her son is [regarded as belonging to the] second generation. From this it is obvious that a child is ascribed to his mother. Said Abaye to him: What then of the following statement of R. Johanan. 'If a man set aside a pregnant beast as a sin-offering and it then gave birth, his atonement may be made, if he desires, with the beast itself, and, if he prefers, his atonement may be made with her young'.  This law would be intelligible if you admit that an embryo is not regarded as a part  of its mother, since this case would be similar to that of one who set aside as a security two sin-offerings,  in respect of which R. Oshaia had stated that a man who set aside two sin-offerings as a security  is to be atoned for with either of them, while the other goes to the pasture.  If you maintain, however, that an embryo is a part  of its mother, the former is like the young of a sin-offering,  and the young of a sin-offering is sent to die!  The other remained silent. 'Is it not possible', the first said to him, 'that there  it is different.  since it is written That are born,  Scripture made it dependent on birth'?  — 'Clever man',  the other replied, 'I saw your chief  between the pillars  when R. Johanan gave the following traditional ruling: The reason  here is because it was written, That are born;  elsewhere, however, the child is ascribed to the father'. What, however, of the following statement of Raba. 'If a pregnant gentile woman was converted, there is no need for her son to perform ritual immersion'.  Why  is there no need for him to perform immersion? Should you reply that it  is due to a ruling of R. Isaac; for R. Isaac stated: Pentateuchally [a covering of] the greater part,  if one objects to it,  constitutes legally an interposition,  and if one does not object to,  no legal interposition is constituted;