The seventeenth perek of Massekhet Shabbat is devoted to a thorough analysis of the prohibition against handling items that are muktze, set aside from use on Shabbat. This halakha has already been mentioned several times in tractate Shabbat, but it has not yet been addressed in a systematic and thorough fashion. The prohibition applies to items that one has mentally set aside from using on Shabbat for some reason, meaning that one assumes he will not use them over the course of Shabbat.
It is not entirely clear why handling items that have been set aside is prohibited. Some have explained that the decree against handling such items serves as a protective measure to ensure that one will not perform prohibited labors on Shabbat. If one may not handle the items used to perform prohibited labors, one will not perform the prohibited labors themselves. Others have suggested that the prohibition is meant to protect against violation of the prohibited labor of carrying from one domain to another. By limiting one’s ability to handle a large number of items, the Sages sought to decrease the likelihood that one would carry items between domains.
Halakhic discussions generally focus on three types of set-aside [muktze] items:
Items set aside because of repulsiveness, items set aside because of function, and items set aside because of monetary loss. The first group includes things that are not generally used or handled, either because they are dirty or because they are repulsive for some other reason. The second group includes objects whose primary function is for an activity that constitutes a prohibited labor on Shabbat. The third group includes items that are generally used only for a particular activity that is prohibited on Shabbat, and the owner of these items is careful not to use them for any other purpose, due to a concern that they will be damaged and he will suffer a financial loss.
The first Mishna in the perek lists utensils that are ordinarily used for activities that are forbidden on Shabbat, but can be used for activities that are permitted on Shabbat. Thus, a hammer can be used to crack nuts on Shabbat and a sack maker’s needle can be used to open a door.