I’m not saying the blood cannot be from her; I’m only asking where we find that if a person checked something, the Sages say maybe you did not check well — and use that leniently. Also, later on the Talmud brings a law about a woman who checked her robe and then lent it to her friend, and blood was found on the robe: the first woman is completely pure, and the second is impure by rabbinic law. And I’m asking: according to Tosafot Rid, it is understandable why the first is not impure by Torah law, because maybe she did not check well, but still the Sages should have declared her impure by rabbinic law, since it is an item that can contract impurity. Unless perhaps one can say that when a person says he checked, we rely on him completely — especially since it is explicit there in the Talmud that ground is easier to check than a garment. And there is also another Talmud passage, on 14a, where Shmuel declares a woman impure by Torah law if she found blood on her thigh that had transferred there from a checked examination cloth belonging to her — specifically a checked one — and he declares her impure by Torah law even though one could say that the check came out clean and the blood on the thigh came from somewhere else, as Rashi explains there.

As stated, the lack of sensation is a flaw. Beyond that, this is a stain and not an actual sighting, and with stains there are various leniencies (like issues involving a louse), unlike laws that apply on the basic Torah level. Especially since they did not decree the laws of stains when they are found on something that cannot contract impurity (and ground cannot contract impurity).

Forgive me, Rabbi, but that doesn’t answer the question. It is explicit there in the words of Rav Ashi that he had to explain Shmuel’s statement as referring to ground in order to declare her completely pure, because if she had been sitting on a cloak she would have been impure (rabbinically). So according to what the Rabbi is saying, why doesn’t Rav Ashi explain it even with a cloak and maintain her presumption of purity? And if the Rabbi will answer that they were strict and decreed in that case specifically where she checked the ground before sitting, because if the ground was unchecked she can attribute it elsewhere — then the fact that she checked the ground only makes things worse, because it is more certain that it came from her body, and therefore Rav Ashi had to explain it as referring to ground. And what the Rabbi says, that this is a stain and not a sighting — what does that mean? The fact that she did not feel anything does not mean for certain that it is not a sighting; maybe it is. For according to Tosafot, sensation is a sign, not the reason for Torah-level impurity. And it could be that in reality she would be impure by Torah law even without a sighting and with less than the size of a gris on the ground, in a case where it definitely came from her body. And according to what the Rabbi is saying, there is no difference at all between something checked and unchecked, because even with something checked one can maintain the presumption, whereas in several passages the law changes depending on whether there was checking or not. Thank you very much for the patience.

I’m not deeply immersed in this passage, and it’s hard for me to answer the details. I’ll just say that I combined several considerations here. There is both a presumption, and also they did not decree regarding something that cannot contract impurity, and also the concern that the checking was not correct. You assume that each one by itself is sufficient, but that is not necessarily the case. It may be that all of them are needed. So they explained it as ground so that it would be something that cannot contract impurity, and only there would the other considerations also take effect.