Which presumption is determinant TB Kiddushinn 79

Today’s daf TB Kiddushin 79 contains a classic discussion. When there is an unknown amount of time lapse, should one use the historical presumption (חזקה דמעיקרא) or the current presumption (חזקה דהשתא) as the determinant factor? Here is the background to better understand the case under discussion. Only a father may marry off his minor daughter (קטנה) and a young girl (נערה) who is between the age of 9 and 12 years old. Once the girl reaches the age of majority (בוגרת), she and not her father may accept kiddushin on her behalf.

Here is the case. Both her father and she accepted kiddushin during the in between time she was a young girl and an adult. Which kiddushin are effective? Rav holds that the current presumption decides which kiddushin is effective while Shmuel says the historical presumption decides which are effective.

“No, it is necessary in a case where he betrothed her on the day that these six months were completed. Rav said: She is a grown woman at present. There is therefore a presumption that since she is now a grown woman, she was also a grown woman when her father betrothed her in the morning. And Shmuel said: Perhaps it is only now that her signs indicating puberty came, but she might have still been a young woman when her father betrothed her.” (Sefaria.org translation)

The Gemara brings three different cases to litigate who is correct, Rav or Shmuel. The first case concerns a mikvah. Originally it was a kosher mikvah containing 40 seah of water. When it was measured sometime later, it no longer contain the minimum amount of water and was no longer kosher. What is the status of all the objects that were immersed in the mikvah between the time it was known to be kosher and the time it no longer was. Do we say the historical presumption or the current presumption is the determinant?

The second case concerns a barrel of wine that was set aside for trumah, the gifts to the kohanim. This barrel of wine was full. As the vintner produce more wine, he designated the percentage of wine needed for trumah to the full barrel of wine. When it was time to give the wine as trumah to the kohanim, it had turned to vinegar. Since vinegar is a different entity than wine, one cannot use vinegar to fulfill the trumah obligation of wine. At one point it was one and a later time it was vinegar. In that in between time when we don’t know exactly when the wine became vinegar, do we say the historical presumption and the farmer has fulfilled his trumah application or the current presumption is the determinant in the farmer needs to set aside other wine to fulfill his obligation?

The third case concerns a gift of money. A healthy person may gift is made to anyone he wishes and this gift is irreversible. A person on his deathbed (שְׁכִיב מְרַע) may later rescinded his gift by becoming well. He only wanted to gift that money because he thought he was good as dead and no longer needed it. What happens in the case were person and his deathbed gives his money away, but sometime later becomes miraculously well. Now that he wants his money back, do we say the historical presumption that he was on his deathbed or the current presumption or that he has recovered is the determinant?

After much analysis and discussion the Gemara decides in favor of Rav. “Mar Zutra said to Rav Ashi: This is what Ameimar said: The halakha is in accordance with the opinion of Shmuel. And Rav Ashi said: The halakha is in accordance with the opinion of Rav. The Gemara concludes: And the halakha is in accordance with the opinion of Rav.” (Sefaria.org translation)