As noted on yesterday’s daf, the Gemara considers a number of cases of forbidden foods in an attempt to clarify whether an issur hana’ah – a prohibition against deriving benefit – is an inherent part of the issur akhila – the prohibition against eating something. One of the cases where we find a disagreement on this matter is gid ha-nashe (the sciatic nerve – see Bereshit 32:33), where Rabbi Shimon rules that we cannot derive benefit from it and Rabbi Yosei HaGelili rules that we can.
The Gemara suggests that Rabbi Yosei HaGelili learns this from a kal va-homer (an a fortiori argument) as follows: We know that the punishment for eating helev (forbidden fats) is very severe (karet), and that the punishment for eating gid ha-nashe is less severe (malkot). Since one is allowed to derive benefit from helev (this is clearly indicated in the Torah – see Vayikra 7:24), then certainly in the less severe case of gid ha-nashe one would be permitted to do the same.
The Gemara asks: And why does Rabbi Shimon, who prohibits deriving benefit from the sciatic nerve, not accept this a fortiori inference? The Gemara answers: This inference can be refuted, as it is possible to say: What is unique to fat? Is it that it is released from its general prohibition with regard to non-domesticated animals, as the prohibition only applies to the fats of kosher domesticated animals. Can you say the same with regard to the sciatic nerve, which is not released from its general prohibition with regard to non-domesticated animals and remains prohibited? Apparently, in some ways the prohibition of the sciatic nerve is more stringent than that of fat.
The expression used by the Gemara to argue against a kal va-homer is ikka lemifrakh – literally, “you can break the argument.” The idea is that a kal va-homer is predicated on the assumption that one law is more severe than another and will remain so in all of its characteristics. If we can find even one instance where that assumption does not hold up, that is to say, if we can find even one case where the assumed hamur (severe case) is kal (lighter) or the assumed kal case is hamur, the kal va-homer relationship is broken. In that case we can no longer extrapolate from one case to the other.