Cessation from all melakha on Shabbat is a positive commandment, as the Torah states: “Six days you shall do your work, but on the seventh day you shall cease” (Shemot 23:12). One who performs melakha on Shabbat not only neglects this positive commandment, but also violates a negative one, as it states: “But the seventh day is Shabbat of the Lord your God; you shall not do any melakha” (Shemot 20:9). If one was warned by witnesses not to desecrate Shabbat, yet did so anyway, he is subject to the punishment of stoning. If he desecrated Shabbat intentionally, but was not warned by witnesses, he is subject to karet (extirpation), as is written: “You shall keep Shabbat, for it is holy for you. Those who profane it shall be put to death, for whoever does melakha on it shall be extirpated (nikhreta) from among his people” (Shemot 31:14). If he desecrates Shabbat unintentionally, he is liable to bring a sin offering (MT 1:1; above 1:14).
Although it is a mitzva to desist from all melakha, the Torah mentions four melakhot explicitly: Ĥoresh, Kotzer, Mav’ir, and Hotza’ah. Ĥoresh and Kotzer are mentioned in Shemot 34:21: “For six days, work, but on the seventh day, cease. At the plowing (ĥarish) and reaping (katzir), cease.” This teaches that even activities upon which human life depends, through which man produces food, are forbidden on Shabbat (Ibn Ezra and Ramban ad loc.). Kindling a fire is also mentioned explicitly: “You shall kindle no fire (teva’aru) throughout your settlements on Shabbat” (Shemot 35:3). The Sages state that this melakha is singled out to teach us that one is subject to punishment for each individual melakha he performs on Shabbat. Thus, if one performs two melakhot unintentionally, he is obligated to bring two sin offerings (Shabbat 70a, following R. Natan; see also below 16:1). The melakha of Hotza’ah is mentioned explicitly as well: “Let everyone remain where he is; let no man leave (yetzei) his place on the seventh day” (Shemot 16:29). Hotza’ah is singled out to make it clear that even though it seems to be an insignificant activity – one merely moves an object but does not change it in any way – it is nevertheless considered a melakha (see below 21:1).
When the Torah prohibits doing melakha, it means creative work, like the melakhot performed when erecting the Mishkan (Tabernacle). Activities that do not create anything new, even if they are physically strenuous, are not prohibited. For example, carrying a needle from a reshut ha-yaĥid (private domain) to a reshut ha-rabim (public domain) is considered a melakha, while moving chairs and tables within the same domain is not (below 21:1); reheating cooked food on Shabbat is not a melakha, whereas cooking raw food on Shabbat is (below 10:2); attaching a window to its hinges is considered a melakha even if it is easy to do, whereas opening and shutting a window is not a melakha (below 15:3); reattaching a broken table leg is considered a melakha, but lengthening a table by adding a leaf designated for such use is not (below 15:7).
We derive a fundamental principle from the Mishkan. Just as the Mishkan was built with intent and planning – “to work with every skilled craft (melekhet maĥshevet)” (Shemot 35:33), so too on Shabbat, the Torah prohibits only melekhet maĥshevet. One who performs a melakha with a shinui (in an irregular manner), unintentionally, lo le-tzorekh gufah (for a different purpose; see below), non-constructively, or without meaning for it to last – has not transgressed a Torah prohibition. In all of these cases, he has not performed melekhet maĥshevet. Nevertheless, most of the above cases are rabbinically prohibited (see sections 3-8 below). The Sages state in the Mishna: “The laws of Shabbat are like mountains hanging by a hair, with few scriptural sources and numerous laws” (m. Ĥagiga 1:8). Indeed, countless laws of Shabbat are based on the melakhot of the Mishkan.
There are numerous further discussions about the shi’urim (measures) that constitute a transgression of a melakha. For example, when it comes to melakhot that relate to food preparation, if one makes a quantity of food the size of a dried fig, he is liable (if the transgression was unintentional, he is liable to bring a sin offering; if intentional, the punishment is death). If the quantity was less than that, even though he has transgressed a Torah prohibition, he is exempt from punishment. In contrast, when it comes to Ĥoresh, Zore’a, Kotzer, and Boneh, even the smallest act renders one liable. In order to keep our subject matter manageable, we will limit our discussion of what one may and may not do to matters of practical relevance.