Niddah

And if you prefer I might say that this is R. Simeon's reason:  He infers the law of the termination of uncleanness  from that of the inception of uncleanness;  as with the inception of uncleanness if it is doubtful whether an object has or has not touched an uncleanness in a public domain it is deemed to be clean, so also with the termination of uncleanness, if it is doubtful whether an object had been duly immersed or not, in a public domain it is deemed to be clean. And the Rabbis?  — What an inference!  There,  since the man is in the presumptive status of ritual cleanness, we cannot on account of a doubt transfer him to a state of uncleanness, but here,  seeing that the man is in the presumptive status of uncleanness, we cannot on account of a doubt release him from his uncleanness. Wherein, however, does this  essentially differ  from the case of an alley of which we learnt: If a dead creeping thing was found in an alley it causes ritual uncleanness retrospectively  to such time as one can testify, 'I examined this alley and there was no creeping thing in it',  or to such time as it was last swept?  — There  also, since there are creeping things from the alley itself and also creeping things that make their way into it from the outside world, the case is the same as one that has two unfavourable factors. And if you prefer I might reply,  This is Shammai's reason:  Because a woman is herself conscious [when she suffers a flow].  And Hillel?  — She might have thought that the sensation  was that of urine. As to Shammai, is there not [the possibility of suffering a flow while] asleep?  — A woman asleep too would  awake on account of the pain,  as is the case where one feels a discharge of urine.  But is there not the case of an imbecile?  — Shammai agrees  in the case of an imbecile. But did he not state, ALL WOMEN?  — [He meant:] All sensible women. Then why did he not merely state WOMEN?  — He intended to indicate that the law is not in agreement with R. Eliezer; for R. Eliezer mentioned 'Four classes of women'  and no more, hence he  informed us [that the law applies to] ALL WOMEN. But is there not the case of stains?  Must we then  assume that we learnt the Mishnah about stains  in disagreement with Shammai? — Abaye replied: Shammai agrees  in the case of stains. What is the reason? — Since she was neither handling a slaughtered bird nor was she passing through the butchers' market, whence could that blood have come?  And  if you prefer I might reply, This is Shammai's reason: If in fact any blood were there  it would have flowed out earlier.  And Hillel?  — The walls of the womb may have held it back.  And Shammai?  — The walls of the womb do not hold blood back. But what can be said for a woman  who  uses an absorbent in her marital intercourse?  — Abaye replied: Shammai agrees  in the case of one who uses an absorbent,  Raba replied: An absorbent too [does not affect Shammai's ruling, since] perspiration causes it to shrink.  Raba, however, agrees  in the case of a tightly packed absorbent. What, however, is the practical difference between the latter explanations  and the former explanation?