1) Mesira is needed because otherwise the person who the Shtar was written for will not have it in his hands as a proof.

Mesira is required, according to the Rosh, in order to acqure the paper of the Shtar:-

2) The Rosh writes (near the end of #4) that, according to Rabeinu Tam, the mesira of the letters is only considered a Maaseh in terms of acquiring the paper of the Shtar itself.

We do learn from the Rosh, at least, that mesira is required in order to transfer ownership on the paper of the Shtar.

The Rosh continues and writes that 2 Kinyanim are required to transfer the actual debt; both writing and mesira:-

3) I have written above that mesira is required according to Tosfos DH Osiyos; either for the simple practical reason that he should have the Shtar is in hands, or so that he acquires the paper of the Shtar. According to both these reasons, the mesira does not effect the transfer of the actual debt. However I have now seen, bs'd, that at the every end of #4, the Rosh writes that mesira is needed for the transfer of the debt itself.

One sees this from a sort of reverse way of reading the Rosh. It goes like this. The Rosh starts by explaining the other opinion, that mesira is the chief Kinyan, but the writing of the Shtar also helps to make the Kinyan. He writes that even though the chief Kinyan is mesira and the Shtar does not achieve anything, nevertheless one also requires the Shtar together with the mesira, in order to make a strong Kinyan to transfer the Shibud.

Then the Rosh writes what is more relevant to us. He compares the opinion that mesira is the main Kinyan with the opposite opinion that writing the Shtar is the main Kinyan. He writes that these 2 otherwise opposite opinions nevertheless have something in common. The common factor is that according to both opinions one requires 2 Kinyanim. Rosh writes that according to the opinion that Osiyos are not acquired by mesira; and the chief Kinyan is the writing of the Shtar because this is a Maaseh; even so one also requires mesira. It follows that according to the opposite opinion mesira is the chief Kinyan but writing the Shtar is also required to strenghten the Kinyan.

For our purposes, it is very important to note that the last 3 words in these very long passage in the Rosh, are "we also need mesira". This proves that mesira is required for the actual Kinyan on the debt, not merely for the paper.

It appears from the Rosh that just as according to the first opinion the secondary kind of Kinyan is necessary in order to provide added strength to the chief Kinyan, so too according to Tosfos (DH Osiyos) mesira is required to add on extra strength to the chief Kinyan of writing.

We learn from Rosh that 2 Kinyanim are required. The main Kinyan is writing the Shtar, but mesira is a secondary Kinyan which is also required in order to strenghten the chief Kinyan of writing.

Dovid Bloom

Source for Rosh from Tosfos 76a DH Ee that a specially strong Kinyan is required for Osiyos:-

1) We saw above that Rosh (end #4) writes that one requires a strong Kinyan in order to transfer the Shibud; which is why both writing and mesira are required.

There is a source for Rosh in Tosfos 76a DH Ee. Tosfos notes the phrase used by the Gemara 76a (3 lines from bottom) "Ad She-Yichtov veYimsor"; that Chachamim say that one cannot transfer Osiyos "until one both writes and does mesira". Tosfos infers from this phrase that one requires a more important Kinyan for Osiyos than for other matters [it seems that Tosfos saw this in the fact that one requires 2 Kinyanim; which is something we do not find in other matters, where one Kinyan is always sufficient to do what is necessary and one does not require a combination of methods.

We can also point out here; in the same way that i wrote above; that it seems that both the writing and the mesira are part of the Kinyan to transfer the Shibud; not that mesira transfers the mere paper. In fact; 2 lines above this; Tosfos writes that mesira is not effective for transferring the paper.

2) The question can now be asked:- why is such a specially strong; double; Kinyan required for Osiyos? The Pnei Yehoshua Kidushin 48a; on Tosfos DH HaMocher Shtar; answers that this is because it is only Miderabanan that a Shtar can be sold. Since Mideoraisa one cannot sell a Shtar; because it is not actually money or property; it follows that one needs both writing and mesira which makes it a very strong Kinyan.

Rabeinu Tam holds that Mideoraisa a Shtar can be sold but even so both writing and mesira are required:-

There is a question that can be asked on the Pnei Yehoshua that I cited above. The Ran Kesubos; 44b in Rif pages; cites the opinion of Rabeinu Tam that the sale of Shtaros is valid even Mideoraisa. Shach Choshen Mishpat 66:1 also writes that in his opinion Mideoraisa one can sell a Shtar [ see Tosfos Kesubos 85b DH HaMocher who writes, to start off with, that according to Rabeinu Tam the sale is only valid Miderabanan but then he questions this. The conclusion of the Ran and other Poskim is that according to Rabeinu Tam the sale is valid Mideoraisa - DB]. The question on the Pnei Yehoshua, is therefore, that we find in the Gemara that one needs both writing and mesira, as Chachamim say in Bava Basra bottom 76a "he does not acquire the Shtar until he writes and does mesira". Rabeinu Tam will not disagree with the opinion of Chachamim. So we see that even though R. Tam holds that the sale of Shtaros works Mideoraisa, he still needs the 2 Kinyanim of writing and mesira. Therefore the answer of Pnei Yehoshua will not help us according to all opinions in the Rishonim.

We will have to try a new explanation why one needs 2 Kinyanim on a Shtar. I suggest, that even though selling a Shtar is possible Mideoraisa, it is still a very novel sort of Kinyan. Clearly one is not only selling the paper of the Shtar; the chief sale is obviously the debt stated in the Shtar. This is considered àéï âåôå îîåï "Ein Gufo Mamon"; the debt is not tangible money at the moment but rather the Shtar is a way of reaching the money later on.

There is an additional problem with selling a Shtar. It seems to conflict with the rule àéï àãí î÷ðä ãáø ùàéðå áøùåúå; "EIn Adam Makneh Davar She-Eino Birshuso"; a person cannot transfer something which is not in his possession [see, for instance, Bava Metzia 16a that if somebody says "I sell you what I will inherit from my father" this is meaningless, but he has not yet received the inheritance]. A debt can be considered as something which he does not yet have control over, since he has not yet received the money.

Therefore, we can understand why, even according to Rabeinu Tam that Mideoraisa one call sell a Shtar, one nevertheless requires a stronger Kinyan than usual, since the fact thst the sale works is less obvious to people. This is why the 2 Kinyanim of writing and mesira are necessary to provide greater strength to the Kinyan.

Dovid Bloom