On our Daf, we are told the generally accepted rule of Rabbi Yehuda that tying any knot which is not permanent is not a biblical transgression on shabbos.
We have seen this idea recently when the Mishna (Shabbos 111b) taught us that one is only liable for tying the type of knot tied by camel-drivers and sailors.
Rashi over there explained that this means that the knot has to last forever (presumably in the absence of human intervention), likes the knots tied to join the broken threads in the curtains of the mishkan.
The Gemara indeed verifies that this refers to the knot which ties a camel’s nose-ring in place permanently, so that the rope that it is driven with can be tied to the ring and untied as needed, as well as the knot tied with a rope to a ship through which ropes can be tied in to order to anchor it.
In contrast, the knots made to tie the anchor-ropes to the knot-rope or the rope to the camel’s nose-ring are not regarded as permanent knots. These are rabbinically forbidden, for reasons given later in the sugya.
On this basis, the Gemara allows tying shoelaces on shabbos under certain circumstances, and forbids them under others, and this sugya needs to be well understood before coming to any conclusions regarding doing this in practice.
Similarly, regarding the melacha of כותב (writing), the Mishna (Shabbos 104b) tells us that one is not liable if one writes with something such a fruit juice or dust that does not last.
Several questions with far-reaching ramifications needs to be addressed, among them:

• What is the reason for this requirement?
• Is this requirement for the effects of a melacha to be permanent, or at least long-lasting, limited to the מלאכות where it is mentioned explicitly by Chazal, like writing and tying knots, or is a general rule for all melachot of Shabbos.
• How long does the effect of the action have to last in order for it to be considered permanent?
  One might argue that having a permanent affect is part of the general rule that an action has to be מלאכת מחשבת, significant and calculated work, and something whose affect is merely temporary is not significant enough to fall into that category.
  On the other hand, one might argue that the way these particular מלאכות were performed in the mishkan were in ways that were permanent, and we derive this rule directly from that, not from the general rule of מלאכת מחשבת. This seems to be the approach that Rashi has taken here regarding regarding tying nots.
  This approach seems logical, given that there are many מלאכות, such as plowing a furrow or baking bread, whose affects are clearly not permanent- for example a furrow is filled in by blowing dust or destroyed by rain or people who walk over it, and a loaf of bread goes rotten and inedible on its own after a few days.
  However, the Magid Mishna (Shabbos 11\15) says explicitly that this is a general rule that applies to ALL מלאכות.
  According to this, one would perhaps need to accept this as a general rule but admit that when a melacha was specifically done in the Mishkan without permanent affects , like in the above two examples, there would be an exception to this rule.
  Alternatively, one would need to limit the definition of “permanent” to the time that these two examples and other like them generally last for- perhaps a week or so (is the lechem hapanim perhaps a precedent for this?)
  In truth, even Rashi who has no need to limit the definition of permanent and clearly has not done so on the Mishna, does seem to understand that the reason why there is still a rabbinical prohibition on tying the rope to the boat or the camel’s ring is because one might leave it there “a week or two.”
  There, Rashi too seems to imply that leaving it there a week or two would be a biblical transgression, and Chazal thus forbade tying it even for a short time in case one comes to do so.
  This apparent contradiction in Rashi requires its own analysis, but we unfortunately do not have time for that now.
  The key to the question of whether the requirement for permanence, whatever it means, applies to all מלאכות, probably lies in an earlier Mishna.
  The Mishna (Shabbos 102) tells us that in order to be liable for a melacha on Shabbos, it has to be דבר המתקים, something that lasts.
  In the somewhat cryptic words words of the Mishna זה הכלל כל העושה מלאכה ומלאכתו מתקיימת בשבת חייב- This is the rule, anyone who does a melacha and his melacha endures on Shabbos, is liable.
  This is the simple meaning of the Mishna, and the way the above-cited Magid Mishna, and possibly the Rambam himself, as we shall discuss later, interprets it.
  In fact, the Yerushalmi, as quoted by the Rashba and the Ritva, also seems to have learnt it that way, as it explains that the construction of the mishkan was considered permanent seeing as it stayed in one place until the divine command to move was given, or that building for a certain period (בנין לשעה) is also considered building.
  However, the wording of the Mishna implies that if the results of the melacha last all of that shabbos, it is considered מלאכת מחשבת, and if they do not, it is not, serving both as the basis for the general exemption of a melacha whose effect is not permanent, and a broad definition of permanent to something that lasts the whole shabbos.
  This means in effect that there is no actual fixed length of time that defines permanent, as it clearly depends when on Shabbos this action is performed- it could be as long as almost 24 hours and as little as less than a minute, an unusual form of measurement to say the least.
  How does this fit in with what we learnt regarding tying knots (and probably also writing) where the examples given were actions that are truly permanent?
  One possibility is that those two melachot are exceptions, due to the specific ways they were performed in the mishkan, but the general rule is far more limited.
  This would also answer the difficulties we raised based on baking, ploughing, and the like, whose affects are not permanent in the classic sense of the world, but certainly last till at least the end of shabbos.
  It would not, however, answer how the Yerushalmi derives this from the building of the Mishkan, which certainly lasted longer than one shabbos, and was not built on shabbos at all!
  Another possibility is that there is indeed no general rule at all, and that this cryptic Mishna has a totally different meaning to what its arguably most simple reading is (certainly the way I first read it.)
  A look at Rashi, shows that he has what’s seems like a rather creative interpretation of both דבר המתקיים and בשבת an approach that is shared by a surprisingly number of other Rishonim with various variations (see for example, Ran , Bertenura on the Mishna, and even the Meiri!)
  He understands מתקיים not to refer to the time that the effect of the melacha lasts, but to the utility of the action- an action that is sometimes good enough to be left as is, and requires nothing to be added, is considered מתקיים.
  He also interprets בשבת not to refer to how long the results of the action need to endure, but rather the day that the action is done.
  This interpretation seems not only creative, but rather problematic. Firstly, the Mishna does not say the words that Rashi uses שכיוצא בו מתקים [בלא הוספה] – (note the brackets indicating a possibly questionable version of his words). Secondly, the word בשבת appears redundant, given that all the melachot we are talking about are referring to things done on shabbos!
  Furthermore, just like the מלאכות of writing and tying were only done in the Mishkan in ways that were long-lasting, it is clear that at least most were done in a way that lasts longer than 24 hours or even a week, so just like the length of the effect of the above melachot needs to be similar to that of the Mishkan, surely all actions need to as well, even if complete permanence of effect is not needed?
  Most of all though, the implication that a melacha needs to produce something which could sometimes be left as is, is rather problematic- since when where the ground herbs in the mishkan left as is? They were used for cooking the dyes. Since when does one leave threshed produce as is without further purifying it or a kneaded loaf unbaked?
  Other Rishonim, take a similar approach to the Mishna, but interpret מתקיים as something which does not need to be undone.
  According to these Rishonim, we are left without an explicit source for a general rule that a melacha which lacks a lasting effect is not biblically prohibited, and it is possible, though not definite, that this leniency is limited to the melachot it has explicitly been applied too.
  I had the gut feeling that the Rambam would learn the Mishna כפשוטו- He does not seem to comment in his פרוש המשניות , or regarding a general rule, but in my online search, I came across the דף על דף anthology who quoted the Minchas Yitchak who claims that the Rambam does precisely this.
  He points to the Rambam )Shabbos 9/13) regarding צובע, where he says that it has to be done with materials that last, but brings the wording of our Mishna in perek 7 with its general rule דבר המתקיים בשבת!
  Why the Rambam chooses to display his interpretation of the Mishna specifically there and not as a general rule, is a question in its own right- however it seems pretty clear that this is how he understands the Mishna, unless he is simply borrowing its wording and applying it to a completely different principle, which while not impossible for the Rambam, would not be our first choice.
  Now that we have seen that the Rambam and some other Rishonim do indeed learn that our Mishna is teaching us a general exemption for a melacha that lacks a lasting effect, and limited the definition of temporary to that shabbos, we need to explain why we treat the melachos of writing and tying a knot differently and require those to have a really permanent effect.
  We also need to reassess whether Rashi and those who interpret the Mishna like him agree with this principle, but simply don’t believe that it is sourced in our Mishna, whether they reject it out of hand, or whether they have a longer view of permanence required for all melachot, similar to what we see by writing and tying a knot, and therefore interpret this Mishna as referring to something completely different.
  The results of the further research required to have massive ramifications for the scope and applicability of this commonly assumed but perhaps narrower than assumed leniency!