The juxtaposition in the Torah of the laws of Shabbat and the Mishkan, the Sanctuary, not only serves to identify the 39 melachot prohibited on Shabbat but also determines the conditions that must exist before one can be held liable for performing a melachah. One of these conditions is intent.
Like the Mishkan, melachah requires “carefully planned work” – melechet machashevet. There are various states of mind that may lack the intent necessary to perform a melachah. In some cases, such a state of mind results in one being biblically exempt from the consequences of one’s act although the act remains rabbinically prohibited. In other cases, the lack of requisite intent means the act is permissible on Shabbat in the first place.
A person who is aware of the act he is performing but forgot that today is Shabbat or that the act is prohibited on Shabbat is called a shogeg. In the Temple era, the shogeg had to bring a sin offering, a chatat, to atone for the act. A person who intended to perform a permitted act, such as retrieving a knife out of a shrub, and in so doing unwittingly performed a different act which is a melachah, such as cutting the shrub when lifting out the knife, is called a mitasek. The mitasek, unlike the shogeg, had no intention of performing the melachah and is therefore entirely exempt.
An act that is permissible in itself on Shabbat but which may – possibly but not inevitably – cause an unintended melachah to occur is called a davar she’ein mitkaven. For example, dragging a garden chair across the lawn, an act permissible in itself on Shabbat, may cause grooves to form in the earth that, if performed intentionally, would constitute the melachah of plowing. Or simply walking on the grass, which is permissible on Shabbat, may result in the uprooting or tearing of grass, which, if performed intentionally, would constitute the melachah of reaping.
Whether or not a davar she’ein mitkaven is permitted constitutes a Tannaic dispute between Rabbi Shimon, who permits it in the first place, and Rabbi Yehuda, who prohibits it. The halacha adopts the more lenient view of Rabbi Shimon. A person cannot, however, claim davar she’ein mitkaven where the melachah was an inevitable result of the permitted act.
For example, if the chair is so heavy that it must form a groove in the earth, or if one washes one’s hands (in itself a permitted act) over one’s own lawn, causing its inevitable watering (constituting the melachah of planting), one cannot claim he did not intend the melachah. This is because the result is so inevitable as to impute to one the intent to perform the melachah in the first place. Such an inevitable result is called psik reishe.
Note, however, that if the inevitable melachah arising from the permitted act is of no use to its performer, such as where one washes one’s hands over a stranger’s lawn, the act is permitted in the first place and is called a psik reishe d’lo neecha lei. Such an act, though biblically permitted, would, according to most opinions, be rabbinically prohibited, unless certain extenuating circumstances exist. Such circumstances are the threat of severe financial loss or when the performance of a mitzvah is involved.
Based on the above principals, Rabbi Moshe Feinstein permits one to open the door of a thermostat-controlled lit oven on Shabbat, even though the resulting intake of air may cause the thermostat to kick in and turn up the flame. Rav Feinstein’s reasoning is that one’s intent is merely to open the oven door. This will not inevitably result in the thermostat kicking in, and it is, therefore not in the category of psik reishe but rather a davar she’ein mitkaven. As such, it is permitted in the first place.