The reason is not that one does not presume that ritual impurity moved from place to place, but because I say that a ritually pure man entered there and took it off the shelf and placed it onto the ground, avoiding the impure garment it would have hit had it fallen. When there is no such explanation, the principle that one presumes impurity moved from place to place does apply, e.g., in the case of the basket where the carcass of a creeping animal was found inside.

The Gemara raises a difficulty: Here, too, let us say that a raven, which often touches creeping animals, came and threw the creeping animal into the basket when it was in the second corner, after the produce had been emptied from it. The Gemara rejects this claim: In the case of a person, who acts with intent, we can say that perhaps a person moved the loaf from the shelf onto the ground. By contrast, with regard to a raven, which does not act with intent, we do not say that perhaps it committed such a purposeful act.

The Gemara raises a difficulty with regard to the ruling of the Tosefta: Now the status of this loaf found on the ground is one of uncertain impurity found in a private domain, and the guiding principle in any case of uncertainty involving impurity in a private domain is that the item with uncertain status is deemed impure. If so, shouldn’t the loaf be deemed impure? The Gemara answers: No, as this is an entity that lacks consciousness in order for it to be asked, and the guiding principle is that with regard to any entity that lacks consciousness in order for it to be asked, whether it is found in a public domain or in a private domain, the item with uncertain status is deemed pure.

And if you wish, say instead: That principle, that in any case of uncertainty involving impurity in a private domain the item with uncertain status is deemed impure, applies to a case of impurity by Torah law, whereas here we are dealing with ritual impurity by rabbinic law. The Gemara adds: The language of the Tosefta is also precise, as it teaches: There was a loaf resting on top of a shelf, and there was an item of light impurity [madaf ] lying underneath it. The term madaf is similar to that which is written: “A driven leaf [niddaf ]” (Leviticus 26:36), i.e., a light item. Likewise, the Tosefta is referring to a light, or rabbinic, impurity.

§ The mishna teaches: And the Rabbis say: The halakha is neither in accordance with the statement of this tanna nor in accordance with the statement of that tanna. The Sages taught in a baraita: And the Rabbis say: The halakha is neither in accordance with the statement of this tanna nor in accordance with the statement of that tanna. It is not accordance with the statement of Shammai, who rules that her time is sufficient and she does not need to be concerned that her menstrual flow started earlier, as he did not enact any safeguard for his statement. And is it not in accordance with the statement of Hillel, who rules that she assumes ritual impurity status retroactive to the time of her most recent examination, as he went beyond [hifriz] his bounds with his safeguard.

The Rabbis continue: Rather, a twenty-four-hour period reduces the time from examination to examination, i.e., if her most recent self-examination took place more than twenty-four hours earlier, she need concern herself with ritual impurity only for the twenty-four-hour period prior to discerning the blood. And from examination to examination reduces the time from a twenty-four-hour period, i.e., if she examined herself in the course of the previous day and discovered no blood, she was definitely pure prior to the examination.

The baraita elaborates: A twenty-four-hour period reduces the time from examination to examination, how so? A woman examined herself on Sunday and found that she was ritually pure, and then she sat through Monday and Tuesday and did not examine herself. And then on Wednesday she examined herself and found that she was impure. In such a case we do not say that she should be impure retroactively from the time of this examination extending back until the time of her most recent examination. Rather, she is impure retroactively for a twenty-four-hour period.

The baraita continues: And from examination to examination reduces the time from a twenty-four-hour period, how so? A woman examined herself in the first hour of a day and found that she was ritually pure, and then she sat through the second and third hours of the day and did not examine herself. And then at the fourth hour she examined herself and found that she was impure. In such a case we do not say that she should be impure retroactively for a twenty-four-hour period. Rather, she is impure retroactively from the time of this examination extending back until the time of her most recent examination, three hours earlier.

The Gemara raises a difficulty: This halakha is obvious. Since she examined herself at the first hour and found that she was pure, there is no reason to render her impure retroactively for a twenty-four-hour period. Why does the baraita state such an obvious halakha? The Gemara answers: Since the baraita taught that according to the Rabbis a twenty-four-hour period reduces the time from examination to examination, it also taught the parallel case, that from examination to examination reduces the time from a twenty-four-hour period, despite the fact that this halakha is obvious.

Rabba says: What is the reason for the opinion of the Rabbis? A woman can sense within herself if she is experiencing a flow of blood. Abaye said to Rabba: If so, her time should be sufficient, as there should be no concern that her flow began earlier. The Gemara explains: And Rabba did not in fact mean this explanation seriously; rather, he wanted to hone the mind of Abaye. The Gemara asks: But if so, what is the real reason for the opinion of the Rabbis?

The Gemara answers: It is in accordance with that which Rav Yehuda said that Shmuel says: The Sages instituted that the Jewish women should examine themselves twice each day, morning and evening. The morning examination is in order to render fit the ritually pure items of the night, i.e., any items that she touched the night before. And the evening examination is in order to render fit the pure items that she touched during the day.

The Gemara continues its explanation: And this woman, since she did not examine herself in accordance with the rabbinic enactment, she loses the status of the ritually pure items she touched over the period [ona] of a day or a night. The Gemara asks: What is meant by: Period of a day or a night? Doesn’t her retroactive impurity status extend back for a twenty-four-hour period? The Gemara answers: It means an additional period of a day or a night, i.e., twenty-four hours in total.

Rav Pappa says to Rava: But occasionally you find three periods of day or night within a twenty-four-hour period. For example, if she examined herself on Monday afternoon and finds blood, then the twenty-four-hour period extending back to Sunday afternoon includes three periods of day or night: Monday day, Sunday night and Sunday day from the time of her most recent examination, as these twenty-four hours do not fit precisely into two such periods. The Gemara answers: The Sages rendered their measures equal, so that one should not differentiate between cases. In other words, they wanted to issue a uniform ruling that applies universally and therefore they established a set twenty-four-hour period of retroactive impurity, regardless of the circumstances.

And if you wish, say instead that they established the set twenty-four-hour period so that the sinner should not profit from his transgression. If the extent of retroactive impurity would be fixed at one additional period of day or night, a woman who remembers and examines herself in the early morning would be impure retroactively for a full twenty-four hours, back to early morning the previous day, whereas one who waits until noon would be impure only for the period of the morning and the previous night.

The Gemara asks: What is the difference between these two answers? The Gemara answers: The difference between them is in a case where she was prevented by outside circumstances and did not perform an examination. According to the second answer she would not be considered a sinner and therefore she would be impure only for an additional period of a day or night. By contrast, according to the first answer her impure status would span twenty-four hours regardless of the circumstances.

§ The mishna teaches: For any woman who has a fixed menstrual cycle, and she examined herself at that time and discovered blood, her time is sufficient, and she transmits impurity only from that time onward. The Gemara asks: Shall we say that the mishna is in accordance with the opinion of Rabbi Dosa, and not in accordance with the opinion of the Rabbis? As it is taught in a baraita that Rabbi Eliezer says: There are four categories of women for whom the halakha is that their time is sufficient: A virgin, i.e., a girl who has never experienced menstruation, a pregnant woman, a nursing mother, and an elderly woman. Rabbi Dosa says: For every woman who has a fixed menstrual cycle, and she examined herself at that time and discovered blood, her time is sufficient, and it is only from that stage that she transmits ritual impurity.

The Gemara answers: You may even say that the mishna is in accordance with the opinion of the Rabbis. The Rabbis disagree with Rabbi Dosa only in the case of a woman who discovers blood at an irregular time, not at the fixed time of her menstrual cycle. But if she discovers blood at the fixed time of her menstrual cycle, they agree with him that her time is sufficient and there is no retroactive impurity. And the mishna is referring to a woman who discovers blood at the fixed time of her menstrual cycle, and therefore everyone agrees that her time is sufficient.

The Gemara raises a difficulty: By inference, one can conclude that Rabbi Dosa says that her time is sufficient even if she discovers blood not at the fixed time of her menstrual cycle. If so, who is the tanna who taught the following baraita? As the Sages taught in a baraita: With regard to a woman who has a fixed menstrual cycle who finds a blood stain, her blood stain is impure retroactively from when the garment in question was laundered. The reason is that if she sees a flow of menstrual blood not at the fixed time of her menstrual cycle, it renders her impure retroactively for a twenty-four-hour period. Therefore, the blood stain likewise renders her retroactively impure.

The Gemara concludes its question: Shall we say that this baraita is in accordance with the opinion of the Rabbis and not in accordance with the opinion of Rabbi Dosa? The Gemara answers: You may even say that it is in accordance with the opinion of Rabbi Dosa, as one can claim that Rabbi Dosa disagrees with the Rabbis only in a case where a woman sees menstrual blood at the fixed time of her menstrual cycle. But if she sees blood not at the fixed time of her menstrual cycle, he agrees with them that she is impure retroactively. And according to this answer the mishna, which deals with a woman who discovers blood at the fixed time of her menstrual cycle, is only in accordance with the opinion of Rabbi Dosa, in contrast to the earlier claim.