Why is it forbidden to purify vessels in a mikveh on Shabbat? There are four answers to this question. Rabba says that it is a decree that one may come to carry the vessels four cubits in the public domain. On a Yom Tov, they decreed not to permit it as it may lead to people doing it on Shabbat. The Gemara cites five different sources to raise a difficulty against Raba’s explanation as there are cases that we do not issue a decree and if we issue in our case, why would they not issue the decree there as well? For each case, it is explained why there is no need to issue a decree - either because it is an uncommon case and the rabbis don’t issue a decree on uncommon cases or because there is no concern that if we permit that action, people will come to think it is permitted in a different case due to unique circumstances. Rav Yosef gives another reason for forbidding the immersion of vessels in the mikveh - a decree lest one squeeze out the water from the object. Rabbi Bibi says a decree lest one delay purifying the vessels purposely for Shabbat/Yom Tov. Rava says it is forbidden as it looks like one is fixing a utensil. If so, why is a person allowed to go to a mikveh? The answer given is that it can appear as if one is going in to cool off. What about dirty water, going in the winter, or on Yom Kippur? The Gemara brings an answer to each of these questions. Regarding Yom Kippur, Rava claims that since (ho’il) it is permitted on Shabbat, it also is permitted on Yom Kippur. The Gemara raises a question against this as regarding a different issue – sipping vinegar to cure a toothache on Shabbat – Rava does not use the “since/ho’il” argument. They conclude that Rava changed his position on the matter of Shabbat. The Mishnah said that it is permissible to put water into a mikvah to purify it but it is forbidden to use an impure vessel and have it become purified in the water. The Mishnah does not seem to conform to the method of Rebbi or the Sages as it appears in a braita that Rebbi forbids in both and Sages permit in both. The Gemara provides a response for how to explain the Mishnah according to both opinions - depending on whether the subject in each source/sentence refers to Shabbat or Yom Tov.