Q) Is it permitted to perform p’sik reishe actions on Shabbos in a situation of tza’ar?
A) First of all, the definition of the term p’sik reish is an “inevitable consequence” – i.e. where performing an action will inevitably result in a forbidden unintentional consequence.
For example, opening a fridge door on Shabbos in a situation where the light will automatically be turned on is defined “p’sik reishe” because opening the door is an action that has a forbidden consequence – i.e. the light turning on.
The Shulchan Aruch (OC 337:1) clearly rules that p’sik reishe is forbidden on Shabbos. However, there are situations in which p’sik reishe d’lo niche lei (i.e. an undesired consequence) is permitted:
In Orach Chayim (Siman 316 Se’if 1) the Shulchan Aruch rules that if one traps a bird on Shabbos into an area in which it is completely trapped – he has transgressed the issur d’orayso of tzod. However, if one traps a bird into a large area in which the bird is not completely trapped – he has only transgressed a rabbinic prohibition. The definition of an area in which an animal is considered trapped is if it would now be possible to catch hold of the animal with one lunge (Mishna Berura s.k. 4).
The Mishna Berura (s.k. 8) adds that if a bird flies into one’s house on Shabbos it is forbidden to close the window even though one’s intention is not to trap the bird, rather to prevent the house becoming cold, because this is a p’sik reishe of tzod. However, if it is cold and one feels discomfort because of the open window then it is permissible to close the window (if one’s intention is not to trap the bird but to protect himself from the cold) in a situation where the tzod would only be forbidden mid’rabonon – i.e. one would not be able to catch the bird with one lunge.
This is because p’sik reishe d’lo niche lei of a melocho d’rabonon is permitted – in a situation of pain or discomfort.