to convey the far-reaching nature of R' Yosei’s statement, as he is lenient in a case of uncertainty whether the eiruv was burnt the previous day or only after nightfall, even though the eiruv is now entirely destroyed. Additionally, the Mishnah taught the case of teruma that became ritually impure to convey the far-reaching nature of R' Meir’s statement, as he is stringent even though the teruma itself is still present, and there is only an uncertainty about when it became impure.
וסבר רבי מאיר ספיקא לחומרא? והתנן: טמא שירד לטבול, ספק טבל ספק לא טבל; ואפילו טבל — ספק טבל בארבעים סאה, ספק לא טבל בארבעים סאה; וכן שני מקוואות — באחת יש בה ארבעים סאה, ובאחת אין בה ארבעים סאה; וטבל באחת מהן, ואינו יודע באיזה מהן טבל — ספיקו טמא.
The Talmud questions the Mishnah’s ruling itself: Does R' Meir really hold that in cases of doubt one must be stringent? Didn’t we learn the following in a Mishnah: If a ritually impure person descended to immerse in a mikveh, and there is doubt whether he actually immersed or he did not immerse; and even if he certainly immersed, there is doubt whether he immersed in a mikveh containing 40 se’a of water, the minimal amount of water necessary for the mikveh to be valid, or he did not immerse in 40 se’a; and similarly, if there are two adjacent ritual baths, one of which has 40 se’a of water in it and is therefore valid, and one of which does not have 40 se’a of water in it, and he immersed in one of them, but he does not know in which of them he immersed; in each of these cases, owing to one’s doubt, he remains ritually impure?
במה דברים אמורים — בטומאה חמורה.
In what case is this statement, which maintains that in cases of doubt one is considered impure, said? It is said with regard to severe forms of ritual impurity, i.e., those imparted by a primary source of ritual impurity.