The beginning of the fifteenth perek discusses the prohibition of tying and untying knots. It differentiates between the knots that are prohibited on a biblical or rabbinic level and knots that are permitted. It is difficult categorise the different types of knots and that task is taken up by the Rishonim. Kehati summarises two of the opinions.
Rashi explains that permanent knots are prohibited from the Torah. Knots that are not permanent, however due to being tied for extended periods appears so, are prohibited rabinically. Whereas knots that will be undone close to their tying (or within the day) are permitted.
The Rif however has a different categorisation that is based on two factors. If the knot is professional and permanent then it is prohibited on a biblically level. If however it is either professional or permanent then it is prohibited rabbinically. If it is neither professional nor permanent then it is permitted to tie.
A fundamental point that is discussed however is, what is the source of the prohibition of tying and untying? We already know that the melachot are learnt from the activities that were engaged in during the construction of the mishkan, but on what particular activity is this melacha based?
Rashi (Shabbat 111b, s.v. ve’elu) explains they would tie the torn threads from the yeriyot (curtains). For the melacha of untying, recall that the chilazon was a creature which was used as the source for techelet dye. Rashi(ibid. s.v. “kach”) explains that the prohibition of untying is learnt from the trappers that would untie knots within their nets in order to adjust its size. The Bartenura here also learns that source for the prohibition of tying is learnt from those that repaired the yeriyot.
R’ Akiva Eiger however has a difficulty with Bartenura’s explanation. The Gemara earlier (Shabbat 74b) asks our very questions. Initially it suggests that it was learnt from those that tied the yeriyot to the anchor pegs. This is rejected as such knots were not permanent and everyone agrees that the melacha relates to permanent knots. The next suggestion is that it is learnt from when they tied torn threads in the yeriyot (Rashi’s source above). The Gemara however rejects this as the source as it can only be a source for tying, but not untying. The Gemara concludes that the source for both tying and untying is learnt from the chilazon trappers. Therefore, why does the Bartenura (and Rashi) learn the prohibition of tying from those that repaired the curtains when the Gemara concludes that both tying and untying are learnt from the chilazon trappers? Furthermore, the Bartenura himself earlier (7:2) comments that both tying and untying are learnt from the chilazon trappers!
The Tifferet Yisrael explains that there are two ways to understand the Gemara’s conclusion. The Gemara was unsatisfied with the yeriyot repairers as a source for both tying and untying as it was only suitable for the melacha of tying. The Gemara then suggests the chilazon trappers. The question is, was the intention that this is the source for both tying and untying or was it meant to be the source for only untying. More fundamentally what difference does it make if the source for tying is the yeriyot repairers or the chilazon trappers?
The Tifferet Yisrael explains that the knot of a chilazon trapper is a professional knot whereas the knots of one the repairs a torn thread in a curtain is… not. Recall that according to the Rif the melacha of tying a knot applies only to a permanent and professional knot. Consequently according to that understanding the Gemara initially suggest that the melacha is learnt from the yeriyot repairers and applies even to regular knots. The Gemara then concludes that the melacha is learnt from the chilazon trappers, and it follows that it only applies to professional knots.
Rashi however holds that whether the knot is professional is irrelevant. The critical issue is its permanence. Consequently the Gemara must maintain its original source for the prohibition – the yeriyot repairers – which relates to even regular knots. The Gemara’s conclusion is not to replace the suggested source for the melacha of tying knots, but to provide another source for the melacha of untying.
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