In Jewish law, in order for food to be considered significant, we ordinarily assume that the person must eat at least a ka-zayit – an olive’s size amount of food. This is true, for example, with regard to the prohibition of eating the gid ha-nasheh – the sciatic nerve. The Mishna teaches that in order to be liable to receive lashes as punishment, the person must eat at least a ka-zayit. At the same time, the Mishna continues and teaches that if the person ate the entire gid ha-nasheh he is liable, even if the gid ha-nasheh was small and did not contain a full ka-zayit.
The Gemara explains the reasoning behind this ruling. Since the nerve is a beriah bifnei atzmah – it is viewed as a significant, free-standing creation – that gives it independent significance with regard to halakha.
Why is the gid ha-nasheh perceived as an independent entity?
Tosafot suggest that once the Torah singles out something as forbidden, it is as if it stated that the Torah specifically forbids that thing, whether it is large or small, as long as the entire thing is eaten. According to this approach, even though only part of the gid ha-nasheh is prohibited by the Torah – on today’s daf Shmuel teaches that only the part of the sciatic nerve that is “upon the spoon (rounded protrusion) of the thigh” is forbidden (see Bereishit 32:33) – nevertheless, since that is what the Torah refers to as the gid ha-nasheh, that is what is considered a beriah bifnei atzmah.
There are rishonim who disagree with Tosafot and require that a beriah bifnei atzmah must have the significance of an entire creation. According to this approach, even though only part of the gid ha-nasheh is prohibited, if the person consumed the entire gid ha-nasheh it is considered a significant act and he would be liable to receive punishment, even though he would not be liable had he eaten only the part of the gid ha-nasheh that was forbidden.