Hullin, Chapter Seven, Mishnah Four
This mishnah deals with whether a thigh that is cooked before the sciatic nerve has been removed is prohibited.
1) If a thigh was cooked together with the sciatic nerve and there was enough [of the nerve] as to impart a flavor [to the thigh], it is forbidden.
2) How does one measure this? As if it were meat [cooked] with turnips.
If one cooks a thigh without removing the sciatic nerve, this turns out to be a case of cooking something prohibited (the nerve) with something permitted (the rest of the thigh). Usually in such cases the determining factor is whether the forbidden substance imparts a taste to the permitted substance. We shall learn more about this principle in tomorrows mishnah.
However, in this case, there is absolutely no way that someone could taste the difference between a thigh cooked with the nerve and one cooked without the nerve. Therefore, we imagine that the thigh was a dish of turnips and the sciatic nerve was meat. How much meat would impart its taste to the turnips? If this ratio exists between the sciatic nerve and the thigh, then the entire thigh is prohibited. If not, he can still remove the nerve and eat the rest of the thigh.