As noted above, the prohibited labors of Shabbat are derived from the work for the Tabernacle, and on this basis, it was determined that only planned, intentional labor is prohibited on Shabbat. This requirement includes several details and stipulations, as enumerated in the following sections. All of these conditions must be fulfilled in order for the labor to be prohibited by Torah law on Shabbat.

A prohibited labor constitutes a desecration of Shabbat only if the person performing the act intended to do it on Shabbat. If he did it unintentionally, he has not violated a prohibition.

For example, it is prohibited on Shabbat to make grooves in the ground, as this is a form of plowing. Nevertheless, if one drags a chair from place to place on Shabbat, and as an inadvertent result of his action the legs of the chair create grooves in the ground, he has not transgressed the prohibition of plowing. The reason is that he intended merely to move the chair and not to create a groove.

Notwithstanding the above, in the case of a permitted action whose prohibited result is inevitable, one is not exempt despite his lack of intent. For example, one who cuts off the head of a chicken on Shabbat cannot claim that his sole interest was the head and he had no intention of killing the chicken. This person has violated the prohibition of slaughtering, because one cannot sever a creature’s head without killing it. The Sages expressed this idea with the phrase, “Can one sever its head [pesik reisha] and it will not die?”

The words pesik reisha have become a halakhic term that is used in many situations. For example, if at the opening of a bottle there is a cotton plug in which wine is absorbed, and one removes that plug, he has performed the labor of squeezing out; he cannot claim that all he wanted was the wine inside the bottle. Since squeezing out the wine from the cotton is an inevitable result of removing the cotton plug, this action is a pesik reisha.

The category of pesik reisha may be divided into two different situations. The first occurs when the person did not intend the inevitable result, but ultimately, he was pleased with the outcome. To return to the above example, although his intent was to access the wine inside the bottle, when the cotton wool was squeezed, he was pleased with that result as well, since it meant that he could also benefit from the wine that was squeezed out. In such a case, he has violated a Torah prohibition.

The second situation occurs when the person is not interested in the prohibited, unintended result of his action. If so, his action is defined as “a pesik reisha that does not please him,” and by Torah law he has not performed a prohibited labor. Nevertheless, the Sages prohibited labors of that kind as well, due to the concern that people might take advantage of this claim even when they are actually pleased about the result, even if only subconsciously.

In order for a labor to be prohibited by Torah law, it must be done for the purpose of its prohibited end result. For example, it is prohibited to dig a hole in the ground, as this is a violation of the labor of plowing. But this prohibition applies by Torah law only when the goal of the person’s action is to create a hole. However, if a person removes dirt from the ground because he wants to use the dirt, and he thereby creates a hole, then by Torah law he has not violated the prohibition of plowing. Nevertheless, the Sages prohibited labor of this type as well.

A labor is prohibited by the Torah only when a single person performs the entire labor from start to finish. For example, if one carries an object from a private domain to a public domain, he has violated a prohibited labor by Torah law. However, if two people together carry an object that can be carried by a single individual, they have violated only a rabbinic prohibition but not a Torah prohibition. If the object could not be carried by a single person, then they would both have violated a Torah prohibition.

The prohibition against performing labor on Shabbat applies only when it was performed in the ordinary manner. If the labor was performed in an unusual manner, it is not prohibited by Torah law.

For example, if a person leaves his home (a private domain) on Shabbat to the street (the public domain) with a toothpick in his mouth, he has violated the prohibition of carrying an object from the private domain to the public domain. The reason is that holding a toothpick in one’s mouth is a usual and acceptable manner of carrying it. By contrast, if one left his house on Shabbat with a needle in his mouth, he has not violated a Torah prohibition, because people do not normally hold needles in their mouths. But here too, the Sages prohibited the performance of labor in an atypical manner.

When the Torah prohibits performing labor on Shabbat, it states, “You shall not perform any labor.” The Talmud infers from the wording of the verse that “performance [of the act] is what is prohibited, but indirect causation is permitted” (Shabbat 120b). In other words, only a direct action is prohibited, not a labor caused indirectly through a different action performed by the person. In this case as well, though, the Sages prohibited the performance of forbidden labors through indirect action.

For example, if a light in one’s house is controlled on Shabbat by an automatic timer, and right now the light is off, is it permitted to move the pins of the clock and to thereby cause the lights to go on sooner than they otherwise would? This is a classic case of an indirect action, as the person would not be turning on the light directly but rather indirectly causing it to go on earlier than planned prior to Shabbat. Such an act is permitted by Torah law, but prohibited by rabbinic law.

It is important to emphasize that the rabbinic prohibition of indirectly causing a prohibited labor to take place does not apply in cases of special need, such as in cases of illness or for security needs. In cases such as these, the Sages did not apply their decree. On this basis, technological devices have been developed in recent years using indirect actions to power certain electric devices to help the sick, the elderly, the handicapped, etc.