The Mishna (Hullin 108) states that if a drop of milk falls on a piece of meat in a stew, and there is enough milk that it can be tasted in the meat, then the meat becomes basar bi'chalav (hereafter, bbh), milk and meat cooked together, and is forbidden. The mishna then goes on to say, that if the milk is mixed into the rest of the pot, then we would have to determine if the milk could be tasted in the entire pot. The question is, when was the milk, in this case, mixed into the whole pot, before or after it made the first piece forbidden?
It is possible to read the mishna (and this is perhaps even the simpler read) to say that it was mixed after the first piece became bbh.. Read this way, the mishna would be teaching that although a piece of meat became bbh, if subsequently it was cooked with other food, to the point that the taste of milk distributed equally and could no longer be tasted in the original piece or the pot, then everything would be permitted. This idea, that a piece of meat that was bbh could become permitted once it no longer had a milk taste to it, is what the Gemara refers to as efshar li'sochto mutar, "if the milk can be [and is] squeezed out [of the meat], then the meat become permitted.
Rashi, however, following the later Gemara, states that the mishna is referring to a case where the milk was immediately mixed into the entire pot. In such a case, the pot takes the place of the single piece in the first case, and we determine if the milk could give taste to the entire pot. There is no indication according to this what the halakha would be if a piece of meat which was bbh lost its milk taste. Would it become permissible again or stay forbidden?
It is reading the mishna with this lacuna which gives the Gemara the opening to discuss what happens to the meat in such a situation. Is efshar li'sochto permissible or forbidden? The Gemara discusses this issue at length, in a complicated sugya, and concludes that according to Rebbe Yehuda, Rebbe, and - among the Amoraim - Rav and Ravina, it is forbidden, and only according to the Sages who argue on Rebbe Yehuda is it permissible. Given this, we rule (Shulkhan Arukh YD 92:2) with the majority and the later opinion that it the piece of meat remains forbidden, even when the milk goes out afterwards. However, Rif does not rule in this case, and Ra'avad thinks that it might be because he rules that in such a case the meat becomes permissible again, and this actually is the Ra'avad's position -efshar li'sochto mutar.
What is the reason behind each side of this debate? It seems that what is being debated here is the core question of how we understand the prohibition of bbh. The Torah forbids us to cook them together, and to eat that which is cooked together. How are these two prohibitions connected? Is the cooking the focus, and the Torah then forbids us to benefit from that item - the bbh - that was the product of the forbidden act of cooking, or is the eating the focus, and the Torah forbids us to even create this thing which we cannot eat? If one focuses on the cooking, then what defines the meat as bbh is the fact that it was - in the past - cooked with milk. If this is the case, then even if the milk is later removed, it does not change the fact that this meat was cooked with milk, and thus it remains bbh - efshar li'sochto assur.
On the other hand, if the eating is the focus, then cooking is just a means to creating a mixture. Looked at this way, the definition of bbh is a mixture of meat and milk. Now, it is true that not all mixtures are problematic. If the two are mixed without heat, even if the meat absorbs the milk, they are not forbidden Biblically. This, however, does not mean that the definition is not about them being a mixture. It merely means that the definition of "mixture" is that they are chemically, not physically, mixed, and this will happen only with the use of heat. If this is the case, then it is quite reasonable to say that if and when the milk separates from the meat, and a mixture no longer exists, then the meat would revert to being permitted once again - efshar li'sochto mutar.
[These two approaches would also lead to different definitions of cooking. Do we mean cooking defined narrowly, or do we mean any form of heat-based transformation, including grilling, frying, and so on. See the Achronim on Shulkhan Arukh YD 87:1].
Now, the answer to this question - whether bbh is about being cooked together or about being a mixture, seems to emerge from our very mishna. Since the mishna states that the meat does not become bbh unless there is enough milk to be tasted, it would seem that something is only bbh if it is a mixture of meat and milk. Thus, if the milk cannot be tasted, it is not considered present, and there is no mixture. This is exactly the point that Abaye makes on this mishna in the immediately previous sugya. From this Abaye also learns that ta'am ki'ikar d'oraitta, that the concept that something's taste is the same as the thing itself is a Biblical concept, because only with this principle can we consider the milk to still be present in the meat. Rava argues with this proof (although not necessarily with the principle itself - see Tosafot, s.v. Amar). Rava states that meat might be bbh not because it is a mixture but because it was cooked with milk. Why, then, do we need the milk to impart its taste to the meat? Because without this, it is not considered that they were cooked together. The reason people cook things with each other is so that they impart their tastes to the dish. If this is not the case, then it cannot be said that a real cooking of the two together took place.
Although we rule that efshar li'sochto assur, this does not definitely prove that bbh is defined by meat and milk having been cooked together, rather than as a mixture of meat and milk. First, Rambam seems to emphasize that eating is the focus, not cooking, and he still rules that efshar li'sochto assur. More to the point, Taz rules (Shulkhan Arukh YD 105, no. 13), in contradistinction to Shakh (ad. loc., no. 17) that if the meat and milk were fully separated, then the meat would revert to being permissible. This is termed by the Achronim as efshar li'hafrido - it is possible to [and one did, in fact] fully separate them, in contrast to li'sochto, which means that one just "squeezed" the milk out, but a tiny bit still remained. Why, according to Taz, does fully separating the two (if and when possible) make it permissible again? Apparently, Taz understands that bbh is defined, even according to this, as a problem of a mixture. However, once something has a forbidden status, it does not lose it easily. As long as some milk - even a miniscule - remains, the status of a mixture continues to adhere to the object. However, when this reality is fully negated, when not even an infinitesimal amount of milk remains, then it is impossible for the status of a "mixture" to continue. In such a case, the meat reverts to being just meat and becomes permissible. Shakh disagrees, and states explicitly that efshar li'sochto assur is based on the idea that the prohibition of bbh is not that of a mixture, but that of meat and milk having been cooked together.
[In closing, it is worth exploring the contrary position of Ra'avad. Ra'avad (on Rambam, Forbidden Foods 9:9) states that the reason to reject the majority opinion and to state - as he does - that efshar li'sochto mutar - is because the position that it was forbidden was only stated in the Gemara according to R. Yehuda (and those who follow him), who is also of the opinion that min bi'mino lo batel, that when one thing is mixed in another, like thing - such as non-kosher meat cooked with meat (they are both meat) - then no nullification occurs. It is hard to understand what this principle has to do with efshar li'sochto, and the Rishonim (see Ramban Hullin 108b) struggle to understand the logic.
It is perhaps possible to make sense of this given what we have said above. R. Yehuda's position that min bi'mino lo batel is usually understood to mean that since everything is the same - it's all meat! - then no nullification is possible. Nullification occurs when there is a difference, and the difference is eradicated. Here, where physically they are the same, there is no way that the majority meat can nullify the minority meat - at the end of the day, it's all still meat. There is, however, another way to understand R. Yehuda (this is somewhat similar to Ramban, but with a different emphasis). It could be that R. Yehuda is stating that the bitul does not work not because it can't, but because the non-kosher meat is actually more powerful in the presence of other meat (a type of matzah min et mino vi'neior - one type found its mate and they "woke up," they reinforced one another). Because everything is meat, then the "conductivity" is better - it is easier for the non-kosher meat to transfer its status to the kosher meat, even if there was only a tiny bit of it. The kosher meat then becomes forbidden because of a status change (this is essentially chatikha na'asit neveilah bi'sha'ar issurim, "the piece becomes forbidden meat - even in non-bbh cases," a topic we will explore in a later post). When, however, it is non-kosher meat mixed with vegetables then the status can only transfer if the forbidden food makes a real-world impact - if its taste is felt in the mixture.
Now, all of this is helpful in explaining why R. Yehuda would say chatikha na'asit neveilah bi'sha'ar issurim, as mentioned above, but it does not seem relevant to the case of bbh. In that case we have two different types of food - meat and milk - so how is his position relevant? Ramban asks this question and does not give a very satisfying answer. Perhaps, though, R. Yehuda's position on mixtures does not let him define the problem of bbh to be that of a forbidden mixture. Since R. Yehuda's approach to mixtures and bitul is based on the transfer of status, and not on the fact that the smaller item does or does not cease to exist, this then determines how he will look at bbh. Basar biHalav for him cannot be a problem of the two mixed together, since neither the meat nor the milk has any "status". Both the meat and the milk are heter, permissible, which means that they are status-less. Thus, if the milk falls on a piece of meat, one cannot say that since it can be tasted it is like it is still there. That is a meaningless idea for R. Yehuda. All R. Yehuda would say is, if it can be tasted, it transfers its status - but it has no status to transfer! Thus, for him, the definition of bbh is that the two were cooked together. In other words, efshar li'sochto assur. However, for the Sages, since min bi'mino is nullified, this shows that nullification operates on the principle of losing or retaining identity, and that when something gives its taste, the principle is that it is still there. Thus, bbh can be based on the mixture of tastes, since when the milk can be tasted, one can say that one has a mixture of meat and milk.]