bava metzia page 7

On the way back from the sea shore, it occurred to me that there is a certain amount of lack of clarity about situations where there is doubt who owns a document;- or at least for me there is a need to see the differences. For in the beginning of Bava Metzia we have a case where there are two people that have found a document, and there is one law. And there is another case of a person who is guarding a document but he forgot if it was for the lender or borrower. Then, there is a case of the lender and borrower holding a document, and the lender claims it was not paid, and the borrower claims he paid it, and there is an argument on pg. 7 about what is the law? Then, there is a case of a document of a loan was found in the street, and it is unclear from who it fell. To Shmuel it is given back to the lender. [R. Yochanan disagrees.] However, this might depend on circumstances for there was a case with the Rosh in medieval Spain where he judged the case based on circumstantial evidence as brought in the Tur Chochen Mishpat, chapter 65 and 66. In any case, why would the case of the lender and borrower holding the document be so different from the case of a third party holding it for them and who forgot to whom it belonged?===========================On the way back from the sea shore, it occurred to me that יש לי a certain amount of lack of clarity about situations where there is doubt who owns a שטר;- or at least for me there is a need to see the differences. For in the beginning of בבא מציעא we have a case where there are two people that have found a שטר, and there is one law. And there is another case of a person who is guarding a שטר but he forgot if it was for the lender or borrower. Then, there is a case of the lender and borrower holding a שטר, and the lender claims it was not paid, and the borrower claims he paid it, and there is an argument on pg. 7 about what is the law? Then, there is a case of a שטר of a loan was found in the street, and it is unclear from who it fell. To שמואל it is given back to the lender. [ר' יוחנן disagrees.] However, this might depend on circumstances for there was a case with the רא''ש, רבינו אשר in medieval Spain where he judged the case based on circumstantial evidence as brought in the טור חושן משפט, chapter 65 and 66. In any case, why would the case of the lender and borrower holding the שטר be so different from the case of a third party שלישholding it for them and who forgot to whom it belonged?