Chullin, Chapter 7, Mishnah 4. This chapter is devoted to the gid hanasheh, the sciatic nerve that the Torah forbids us to eat. Until now the mishnayot have dealt with the prohibition itself and where it applies. Our mishnah moves to a new question: what happens when the gid hanasheh is cooked together with the meat of the thigh, so that its flavor spreads into food that is otherwise completely permitted?
To understand the mishnah we need a principle that governs a great deal of the laws of kashrut: taam ke'ikar, the flavor of a food carries the status of the food itself. When a forbidden item is cooked with a permitted item and the permitted item absorbs the taste of the forbidden one, the permitted food becomes forbidden along with it. Eating it would mean eating the flavor of something the Torah prohibited. There is a machloket among the authorities whether taam ke'ikar is a Torah level law, de'oraita, or a rabbinic enactment, derabbanan.
The rule operates even where both items taste exactly alike. Picture a single pot holding two cuts of meat, one permitted and one prohibited. To a human palate, the taste that seeped from the one into the other is indistinguishable from what was already there, yet halachah still counts it as a transfer. Taste that originated in something the Torah barred now sits inside the permitted piece, and that by itself is enough to prohibit it.
The rule has a boundary, though. Where the permitted component is overwhelmingly large, whatever came from the prohibited item is spread so thin throughout the pot that it counts for nothing. Here bitul takes over: the prohibited item is treated as nullified, and what remains may be eaten.
The Thigh Cooked with the Gid Hanasheh
With this background, the mishnah takes up its case: "yerech shenitbashel bah gid hanasheh", a thigh that was cooked while the gid hanasheh was still inside it. "Im yesh bah binoten taam, harei zo asurah": if the gid hanasheh is present in a quantity capable of imparting flavor, the entire thigh is forbidden, because during the cooking the flavor of the gid was absorbed into the meat around it.
But here we run into a practical difficulty, and the mishnah asks it directly: "Keitzad mesha'arin otah?" How do we measure this? The gid hanasheh tastes essentially like the meat it sits in. We cannot taste the pot and determine whether a transfer of taam took place. So how is the assessment made?
Meat and Turnips
The mishnah's answer is "kebasar belefet", like meat cooked with turnips. We construct a parallel case in our minds using two foods whose flavors are clearly distinguishable. Imagine a piece of meat the same size as this gid hanasheh, placed together with a quantity of turnips equal in volume to this thigh. The proportions of the imagined case match the proportions of the real one exactly: meat corresponds to the gid hanasheh, and turnips correspond to the thigh.
Then comes the question: cooked together, would those turnips pick up a meaty taste, or would they not? If they would, the actual thigh is judged to have taken in taste from the gid hanasheh, and it may not be eaten. If, however, the thigh is so substantial that a piece of meat of that size could not flavor turnips of such volume, no taam crossed over at all. The gid is nullified within the surrounding flesh, and the thigh is permitted. The following mishnah will carry this discussion further.
The Ratio of One to Sixty
According to many authorities, this is the source of the famous measure of one part in sixty. When we say that the meat is to the turnips as the gid hanasheh is to the thigh, we are describing a thigh so large in relation to the gid that no flavor is conveyed. That point is reached at a ratio of one to sixty: one part forbidden against sixty parts permitted, and the mixture is permitted. If the permitted food falls short of sixty parts, say a ratio of one to fifty nine, we do not consider the forbidden item nullified. We assume the flavor was in fact absorbed, and the mixture becomes prohibited. From this case of the gid hanasheh and the thigh, then, we derive the ratio of one to sixty that permits a mixture.