If one wishes to engage in a permissible activity, but in the course of doing so might also perform a prohibited action, he may undertake the permitted action on two conditions: that he does not intend to perform the action, and that the permitted action will not inevitably result on the prohibited action. Thus, one may drag a bed or couch on the ground on Shabbat even though the legs will probably create furrows in the ground (an act of Ĥoresh), since the furrows are made unintentionally and it is not certain that furrows will be created at all. Similarly, one may walk on the grass even if grass will likely be uprooted, because he does not intend for this to happen, and it will not necessarily happen at all. This type of situation is called davar she-eino mitkaven (an unintended act).
However, if it is clear that by dragging the bench a furrow will definitely be made in the ground, then one may not drag the bench (below 19:2). Similarly, if it is clear that by walking in a particular area, grass will definitely be uprooted, one may not walk there (below 19:8). As long as it is clear that during the performance of a permitted action, a prohibited one will definitely be performed, one cannot maintain that he did not intend for it to happen. Rather, it is as if he intentionally performed the prohibited action. However, it is only a Torah prohibition if one is interested in the result of the forbidden action; if he is not interested in the result, the prohibition is rabbinic.
The Sages tell further us that one may not close the door to his home when there is a deer inside (Shabbat 106b). He cannot claim that he wished only to close the door but not to trap the deer, since it is clear that by closing the door he has trapped the deer. Thus, it is as if he intentionally trapped it. This scenario is akin to one who cuts off the head of a bird so a child can play with it claiming that he did not intend to kill the bird but only cut off its head. This claim is insubstantial; it is clear that he meant to kill the bird, an act that is prohibited by the Torah. About this paradigmatic example the Talmud asks rhetorically: “psik reisha ve-lo yamut?” – “Its head is detached – will it not die?” (Shabbat 75a; MT 1:5-6; Kesef Mishneh ad loc.). This principle has thus come to be known as “psik reisha.”
These principles are not exclusive to Shabbat. Rather, regarding all Torah prohibitions, the rule is that davar she-eino mitkaven is permitted, while psik reisha is forbidden.
. In R. Yehuda’s opinion, davar she-eino mitkaven is rabbinically prohibited, which means that even if it is not certain that a melakha will be performed, and the person’s intent is to perform a permitted action, it is still forbidden if the person would benefit from the unintentional melakha. In R. Shimon’s opinion, since he intends to perform a permitted action, and it is not certain that a melakha will accompany this action, it is permitted (Beitza 23a). In the Gemara, Rav follows the opinion of R. Yehuda while Shmuel follows R. Shimon. Abaye states in Rabba’s name that the halakha follows R. Shimon, and a davar she-eino mitkaven is permitted (Shabbat 22a), even in the case of a Torah prohibition (Shabbat 46b). This principle – that we follow R. Shimon’s opinion – applies to all Torah prohibitions. This is also the opinion of Tosafot (Shabbat 75a, s.v. “mitasek”), Rashba, Rosh 14:9, and Ran. However, She’iltot maintains that the halakha follows R. Shimon only in the case of Shabbat, because on Shabbat only melekhet maĥshevet is prohibited by Torah law (quoted in Tosafot to Shabbat 110b, s.v. “Talmud”).When it is almost certain that the prohibited action will take place, some still permit performing the permitted action because the forbidden result still falls under the category of davar she-eino mitkaven (Me’ iri, Ritva, Rivash); others maintain that in such a case it is rabbinically prohibited (Terumat Ha-deshen §64, Maharsha quoted in BHL 277:1, s.v. “shema”). If it is clear that the prohibited action will occur barring unforeseen circumstances, all agrees that it is forbidden by Torah law.
Psik reisha on a rabbinic prohibition is forbidden according to most poskim. Thus one may not drag a bench that would definitely make a furrow, even though the furrow is being made with a shinui, which means the prohibition is intrinsically rabbinic (MA 314:5, and also implied by Tosafot, Ramban, Ritva, Rosh, and SA 337:1; this is also the opinion of Eliya Rabba, Tosefet Shabbat, Gra, SAH 337:1 and 3, Ĥayei Adam 30:2, R. Akiva Eger, Ben Ish Ĥai, Year 2, Va’era 6, MB 314:11 and 316:18, and SHT ad loc. 21). However, some rule leniently in such a case (Terumat Ha-deshen, Me’ iri, Maharsham). Their opinion is sometimes taken into account when there are other reasons to be permissive as well (see Yabi’a Omer 4:34; Menuĥat Ahava 2:1:6).
Psik reisha de-lo niĥa lei – where the unintentional result is not desired: According to Arukh and several other Rishonim, davar she-eino mitkaven includes performing a permitted action that will inevitably result in a melakha, if one is unhappy with (or indifferent about) the result of the melakha. However, we do not entertain this possibility in the case of cutting off a bird’s head. Even if he claims that he is not pleased with the bird’s death, either we do not believe him, or we regard his outlook as null given how most people think. But when the result is really unwanted, according to Arukh it is considered a davar she-eino mitkaven and is permitted. The vast majority of poskim maintain that such an act is still rabbinically prohibited, but Arukh’s position can be used to justify ruling leniently when there are additional reasons to be lenient as well.
Psik reisha de-lo niĥa lei on a rabbinic prohibition: In such a case, there is a difference of opinion among the poskim, and as a result, we are lenient in times of need (MB 316:5, 321:57; see Yabi’a Omer 5:27:1, which maintains that the majority of poskim permit this). In any case, this is an example of shvut di-shvut, and we are lenient in such a case for the sake of a mitzva or a great need (see sections 11-12 below). Psik reisha de-lo niĥa lei in a case of a double rabbinic prohibition is permitted le-khatĥila according to most poskim (Pri Megadim, Eshel Avraham 316:7; Dagul Mei-rvava §340; below 18:3:2; however, it seems that MB 340:17 is stringent).
Safek psik reisha: May one shut the door to his house when he is unsure if there is a deer inside? May one open a refrigerator when he is unsure whether the refrigerator light was disabled before Shabbat? Some poskim permit such actions, categorizing them as davar she-eino mitkaven (Taz 316:3); others forbid them, as the safek concerns an already-existing situation, and one must be stringent in the case of a safek de-Oraita (R. Akiva Eger). However, when necessary we are lenient. If the prohibition is rabbinic, one may certainly be lenient, as we are with rabbinic prohibitions in cases of uncertainty (see BHL 316:3, s.v. “ve-lakhen”).
There are certain actions whose permitted or forbidden nature is determined solely by the intention of the person performing the action. If he intends to perform a forbidden action it is forbidden, while if he intends to perform a permitted action it is permitted. See below 15:10 for the case of aromatic branches, and see Harĥavot 9:5:9 regarding filling a hot pot with cold water.