Niddah

MISHNAH. THE SAGES SPOKE OF A WOMAN IN METAPHOR: [THERE IS IN HER] CHAMBER  AN ANTE-CHAMBER  AND AN UPPER CHAMBER.  THE BLOOD OF THE CHAMBER  IS UNCLEAN, THAT OF THE UPPER CHAMBER  IS CLEAN. IF BLOOD IS FOUND IN THE ANTE-CHAMBER, AND THERE ARISES A DOUBT ABOUT ITS CHARACTER,  IT IS DEEMED UNCLEAN, BECAUSE IT IS PRESUMED TO HAVE COME FROM THE SOURCE. GEMARA. Rami b. Samuel and R. Isaac son of Rab Judah learnt the tractate of Niddah at R. Huna's. Rabba son of R. Huna once found them while they were sitting at their studies and saying: The chamber is within, the ante-chamber is without and the upper chamber is built above them,  and a duct communicates between the upper chamber and the ante-chamber.  If blood is found anywhere from the duct inwards, and there is any doubt about its character,  it is deemed unclean  but if it is found anywhere from the duct outwards, and there is a doubt about its character,  it is deemed clean.  He  thereupon proceeded to his father and said to him, 'You told them, Master,  that "if there is any doubt about its character  it is deemed unclean", but have we not learnt: BECAUSE IT IS PRESUMED TO HAVE COME FROM THE SOURCE?'  'I', the other replied, 'meant this: [Blood found anywhere] from the duct inwards is  undoubtedly unclean,  [but if it was found anywhere] from the duct outwards, it is deemed to be doubtfully unclean'.  Said Abaye: Why is  it [that if blood is found anywhere] from the duct outwards it is deemed to be doubtfully unclean?  Obviously because it is possible that she bowed down and the blood flowed thither from the chamber. [But, then, why in the case where blood is found anywhere] from the duct inwards, is it not also assumed that she might have staggered backwards  and the blood originated from the upper chamber?  Rather, said Abaye, if you follow possibilities  the uncleanness is doubtful in either case  and if you follow presumption [blood found anywhere] from the duct inwards is undoubtedly unclean,  [but if it was found anywhere] from the duct outwards it is undoubtedly clean. R. Hiyya taught: Blood found in the ante-chamber  renders [the woman] liable [for a sin-offering] if she enters the Sanctuary,  and terumah  must be burnt on its account.  R. Kattina, however, ruled: No sin-offering  is incurred if she enters the Sanctuary,  and terumah  is not burnt on its account.  According to the first alternative  which Abaye mentioned, viz., 'If you follow possibilities',  support is available for the ruling of R. Kattina  but  a divergence of view is presented against R. Hiyya. According to the second alternative  you mentioned, viz., 'If you follow presumption'  support is provided for the ruling of R. Hiyya