Rav Shach means to answer why we do not learn from Sota [laws of Sota perek I ] to say even a doubt about things [if they are clean or not] are clean in a public domain. I asked on his answer a few days ago, and it occurred to me today that my question was based on a misunderstanding of what Rav Shach was saying. What he is saying is this. If we would learn from a sota to a public domain, then in fact even things that have a prior status of being unclean would be clean. It would be because of gezerat hakatuv. But in fact we don't. We only learn from sota in a  private domain that a doubt is considered to  be definite.] And this explains the Gemara that otherwise is hard to understand. The Gemara says the sages learn from Sota to say a doubt of uncleanliness in a private domain is definitely unclean. Then it asks then why not learn from Sota in a public domain. [i.e.the sages hold a mikve that was measure and found lacking the right volume, all things made on it whether in a public or private domain are unclean.-So the question is why not say in a public domain they are clean?] Answer: the mikve is not like a sota. The mikve is a doubt because of the volume. The question about volume is not connected with a private or public domain. The doubt about the sota is because of being alone with a man in a private domain. There is no privacy in a public domain. Then the gemara asks but even so is it not so that every doubt about cleanliness  a public domain is pure/ Answer it is a case of two things pushing towards uncleanliness. The status of uncleanliness of an object because it has been made clean--or or the person that is handling  the object. Plus the present status of the mikve which was measured and found to be lacking the proper volume. This gemara is very hard to understand. At first it says the reason the doubt for the mikve is unclean is because we learn from Sota. Then it seems to change its mind and says  the reason is because two hazakot against one. Now however we understand the gemara perfectly because it is not learning from sota about anything in a public domain. Only the private domain. As for the public domain we learn nothing from sota at all. Things are clean in a public domain and that would include sota if they have no prior hazaka at all. Not one clean or unclean. But if anything would have a hazaka of being unclean it would be unclean.

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