The Gemorah on Daf 149a brings a Tosefta that says one shouldn't look in a mirror on Shabbos lest they come to go get a pair of scissors and take out a hair that they see. R' Meir is matir however if the mirror is kavuah because when the person goes to go get the scissors they will remember that it's Shabbos.
We see many times that chazal made gezeras on Shabbos not to do something in order not to come to be ovar an issur. Why is it in this case that according to R' Meir at least the person will remember that it's Shabbos in the process and won't be ovar the issur. Why is it that in other cases we don't say this savarah. Unfortunately I'm lacking a clear example to ask a kasha on (as to why we don't apply this savarah else where) So really my question is first why davka in this case and 2nd what are other cases where we see chazal didn't want to apply this savarah and that the gezera still stands even though there would be makom to say that the person would remember in the process of about to do an issur the person would remember (I thought perhaps playing musical instruments since if it breaks one might come to fix it however in that case you could say the person would be able to use his bare hands to fix the instrument and wouldn't have the possibility of saying that as he is go
ing to get the cli he'll remember it's Shabbos...)
I hope my questions are clear as I wrote them...
Yehoshua, Yerushalayim, Eretz Yisrael
Yehoshua, this is another very interesting question!
1. See the Gemara in Beitzah (beginning of 3a) which states that the reason why one may not eat fruit which fell off the tree on Shabbos is a Gezeirah that one might climb up and pick fruit off. Tosfos there (DH Gezeirah) writes that the concern that one might pick them off applies only to fruits on a tree but not to turnips and raddishes for which one requires a spade to dig them out of the ground. Before a person manages to get the spade he will remember that it is Shabbos, since it is not easy to dig them out. There is a text of Tosfos, printed on the side of the Gemara there in the name of the Maharsha, that states that we are concerned only that a person will climb up the tree and pick off grapes and fruit or squeze out their juice because he has a strong desire for these fruits and juices.
2. Tosfos continues and writes that Rabeinu Eliezer of Metz ate on Shabbos food which a Nochri had roasted on Shabbos for himself. There is no concern that if we allow someone to eat food roasted by a Nochri that he will come to roast it himself, because since roasting involves considerable effort the person will remember that it is Shabbos and will stop himself from transgressing.
3. A similar distinction is made by the Teshuvos Radbaz (2:290), cited by the Sha'arei Teshuvah (OC 307:1). The Radbaz writes that is forbidden to read a book on Shabbos with the intention of finding places in the book which require corrections. However, the reason for this prohibition is not that one might come to write on Shabbos, because this is not similar to the prohibition that Chazal made (Mishnah Shabbos 11a) that one may not read to the light of a candle lest one tilt the candle to make the oil burn better. It is very easy to tilt the candle, which is why Chazal made a Gezeirah, while in contrast if a person decides to go and get a quill and ink to make a correction he will remember in time that it is Shabbos today. Instead, the Radbaz writes that the reason why one may not check the book for mistakes is that this is similar to the Halachah that one may not check one's fields and properties to see what work needs to be done there tomorrow (see Shulchan Aruch OC 306:1).
4. We see from the above examples that Chazal made a Gezeirah only where it is easy to stumble and transgress the prohibition, but not where effort is required to do the Melachah. I agree with your logic about the musical instruments. The Mishnah states in Eruvin (102b) that one may not tie the string of a harp. One sees from there that a Torah prohibiton can be transgressed very easily with musical instruments, which is why Chazal made a special Gezeirah that one may not play them.
Kol Tuv,
Dovid Bloom