Here goes: The part that most people know is that we do not wear tefillin because, in the Torah, tefillin are called ‘os’ (a sign, i.e. between Hashem and Yisrael), and Shabbos and Yom Tov are called ‘os’ as well. So what? The Torah considers it to be a “slight” to the ‘kavod’ (honor) of Shabbos/YT to suggest that we need the ‘os’ of tefillin when we already have one in the day itself! [Ed: Many have heard the concept that two witnesses are required to testify in Jewish court, and we actually have a third ‘os’ - the ‘bris’ (circumcision – hey, this week’s parsha!). The point is that we must constantly have two “witnesses” at all times. On Shabbos and YT, since we have two already, the third - tefillin – is not necessary. I believe that while that explanation is true and very compelling, it is not the primary one, and the one quoted by the Mishnah Berura is the one mentioned above.]
Anyway, so we shouldn’t wear the tefillin on an ‘os’ day, so as not to slight the day. Now actually, it is only a ‘slight’ to Shabbos/YT if we wear the tefillin for the sake of the mitzvah. However, if we wear it “not for the mitzvah”, there is no slight! (Also, neither is there a violation of ‘bal tosif’ – Do not add to mitzvos, i.e. the prohibition against doing more than the Torah commands, as if it were a mitzvah – if it is done in a non-mitzvah way.) It emerges, then, that there is no prohibition, according to the Torah, against wearing tefillin on Shabbos/YT. It was the rabbis who came along and prohibited the wearing of tefillin. According to some authorities, they prohibited it only in public. According to most, however, the prohibition was enacted “across the boards”, partly for fear that one would remove them while wearing them in the ‘reshus harabim’, and thus violate the melacha of ‘hotza’ah’ – carrying.
Where are we going with all this?? What if you are walking in the street or in the field on Shabbos, and you chance upon a pair of tefillin lying, unprotected? We are commanded to care for all mitzvah objects! Based upon the above analysis, we understand the halacha now: pick them up, put them on, and wear them to a safe place! You are not violating any Torah law by wearing them, because you are not wearing them for the sake of the mitzvah. You are not carrying them, because they are an adornment, not a burden. The rabbis waived their prohibitions (tefillin, possibly muktzah) in this case out of deference for the care of mitzvah objects! Make sense now?
One more thing: according to our discussion today, are tefillin, in fact, ‘muktzah’? What if I accidentally left them on a table that I need cleared, or in harm’s way, may I move them within my house?
Great question! There is a slight difference of opinion in this matter: According to some, they are, in fact, not muktzah at all and may be moved for any reason – either for their safety of for your needs. According to others, they are muktzeh in the category of ‘kli she’melachto l’issur’ (a utensil whose primary use is prohibited), just like a hammer. The halacha on the latter category states that while they may not be moved for their own sake, they may be moved if you need the place they occupy!