As I was coming back from the sea, it suddenly occurred to me why the Rambam would not hold with the law of R. Isaac. [R. Isaac said the lender who holds a guarantee is considered to own the mashkon.] The reason is in the Shach in Choshen Mishpat, chapter 72, law 2. There the Shach shows that to have possession of the mashkon means the lender is considered to a a borrower who is obligated to pay if the mashokn is lost, even if it was lost in extreme circumstances. This he shows is the approach of Rashi. However, we know the Rambam does not hold that way. I assume the reason is the Mishna itself brings the opinion of R. Akiva that the lender with the mashkon is a paid guard. So, the Rambam does not hold like Rashi. And as the Shach shows, the only way to understand R. Isaac is that the lender is considered a borrower. So the Rambam does not hold with R. Isaac.----------------------------------------------As I was coming back from the sea, it suddenly occurred to me why the רמב''ם would not hold with the law of ר' יצחק. [ר' יצחק said the lender who holds a guarantee is considered to own the משכון.] The reason is in the ש''ך in חושן משפט פרק ע''ב הלכה ב'. There the ש''ך shows that to have possession of the משכון means the lender is considered to a שואל who is obligated to pay if the משכון is lost, even if it was lost in extreme circumstances (אונסים גדולים כגון טבעה ספינתו) . This he shows is the approach of רש''י. However, we know the רמב''ם does not hold that way. I assume the reason is the משנה itself brings the opinion of ר' עקיבא that the lender with the משכון is a paid guard. So, the רמב''ם does not hold like רש''י. And as the ש''ך shows, the only way to understand ר' יצחק is that the lender is considered a שואל. So, the רמב''ם does not hold with ר' יצחק.