Dear Rabbi Kornfeld, Shlita
The following question came up in the shiur. Perhaps you can help with the answer.
On Shabbos 74b, the Gemara is trying to find the precedent in the Mishkan for "Kosher", tying knots. It rejects the tying of the ropes to the pegs when erecting the Mishkan, because those are only temporary knots.
Presumably the precedent for "Boneh" is the erecting of the Mishkan, which was just as temporary. However, the erecting of temporary buildings is also "Boneh". Why do we not conclude that temporary knots are also prohibited as "Kosher" from the fact that temporary knots were tied to erect the Mishkan?
There is an argument among the Rishonim regarding exactly what we learn from the Mishkan in regards to Shabbos. The main opinion seems to be that of many Rishonim, such as the RITVA and CHIDUSHEI HARAN, who understand that only the types of Melachah done in the building of the Mishkan are forbidden on Shabbos. However, the parameters (i.e. what type of tying, what type of building) of the Melachah are in fact set up by Chazal. This is also the opinion of the TESHUVOS HAREMAH (#119).
According to the Ritva's opinion, Abaye is not saying that this type of tying is not a Melachah. Rather he is saying that Chazal did not call this a Melachah which therefore means that it cannot be the source of the Melachah in the Mishkan. (This also seems to be how RASHI learns Abaye's question, see DH "Kosher".) As Chazal decided (how they decided is another discussion) that even a small temporary building is called Boneh (see Shabbos 102b), small temporary structures could be the source for the Melachah of Boneh in the Mishkan.
[To see a different opinion, see the AVNEI NEZER (#211) who based on a TESHUVOS HAROSH says that the Shiurim of the Mishkan were also learned from what was done in the Mishkan].
With regard to Boneh, the Mishnah (102b) says one is only Chayav only if he builds something that is used as is , i.e., it can be used for any amount of time as is. A temporary structure, which is not built to last, is prohibited only mid'Rabanan (Beitzah 32b).
The Mishkan was erected in a manner in which it could have remained in place for any amount of time. Sometimes it remained for 20 years! However, the knots were not tied in the normal fashion. They were tied in a way that it would be easy to undo them, since if the ropes had been knotted with strong knots it would have been very hard to unknot them. Since temporary knots are permitted on Shabbos, obviously we did not learn the Melachah of knotting from those knots.
As for your question of why we did not learn the Melachah of knotting from those knots, the answer seems to be as follows. The criteria Chazal received from Sinai for defining Melachos of Shabbos are "Anything that was performed in the Mishkan and was Chashuv ." A Melachah that is not Chashuv cannot be a prohibited Melachah on Shabbos, mid'Oraisa (Shabbos 96b).
Apparently, a temporary structure, or knot, cannot be considered a "Melachah Chashuvah."