When I lived in Israel, I was once invited to Shabbat dinned at the home of a young Yemenite rabbi, of the type that follows the rulings of RAMBAM completely (there are five distinct Yemenite groups, with different ideologies and practices) When his wife served us dinner, I was shocked to see on my plate fish and meat together. "Isn't this forbidden?". I asked. If one goes to almost any observant Jewish home, fish is NEVER mixed with meat In fact, after the fish course is served, clean plates and forks are brought to the table, People eat some other foods in between, and also drink; most often a bit of spirits. In some homes, water is brought to the table, and each person washes his hands before proceeding to the meat or poultry course. Here were sitting on my plate fish and meat together. It seemed to me almost as if I was seeing toast on a Seder plate. He looked at me, and said "I never even heard of that!". We spent the rest of the evening pouring over rabbinic sources, spanning the course of seventeen hundred years. The primary text here is a brief passage in the Talmud (Pesachim 76b) that eating fish (binita) together with meat causes "Davar Acher" ("something else", a euphemism for leprosy.) The classical commentaries on the Talmud discuss whether the prohibition is across the board, including all kinds of fish, or only the binita fish, whose identity is unknown, except that it was tiny. Most understood that the passage refers only to the two being cooked or roasted together, rather than merely on the same plate. However, the entire idea is apparently contradicted by another section of Talmud (Hullin 111b) which discusses at length the question if fish is cooked with dairy utensils, when can it or can it not, subsequently go into a meat dish without violating the meat-milk prohibition.. There is no mention in Hullin of the "danger" of eating fish with meat. We found that RAMBAM doesn't even mention the topic. But was this because of his view that we ignore the medical and scientific information in Talmud, or because, as has been suggested by many commentators, he "knew" that the binita fish is now extinct, and that the human body had changed so much in the intervening years, that even if we did have it, it would no longer effect us? (Chatam Sofer). We looked trough the commentaries on the Shulchan Aruch. Some were silent. Some proposed that the "prohibition" was only for a certain time period. Some (TAZ) considered this combination so deadly, that it is not nullified in any amount (rather than the usual one in sixty), as it is to be treated like a venomous poison, as "sakanta adifa mi'Isura" (danger is to be treated more severely than halachic prohibitions). Recent and contemporary halachic opinion is likewise divided. One of my sons works as a kashrut supervisor for an OU affiliated agency. He tells me that their policy is to follow TAZ, and treat the fish and meat combination as more severe than any halachic norm. On the other hand, some halachic decisors rule that there is no prohibition, but only a custom. Therefore, if the combination isn't recognizable, there is no prohibition, or even custom. Worcester sauce, which contains minute amounts of fish, is, according to these poskim, perfectly acceptable to eat with meat. (Rabbi Yitzchak Abadi, among others). A similar, albeit post Talmudic issue, is that of fish with milk. Rabbi Yosef Karo, the author of the Shulchan Aruch, writes (in his magnum opus Beit Yosef), that this combination is dangerous, and hence prohibited. Virtually all commentaries state the opinion that this was a misprint, and should have read "fish and meat". Nevertheless, one commentator (seventeenth century) states that he asked a physician, who told him that "there is no deadlier combination than milk and fish". Many point out, however, that we see people eating fish with milk all the time, with no ill effects. Today, this "prohibition" is observed by many Sepharadic and Hassidic Jews, primarily out of respect for Rav Yosef Karo.. The late Rav Ovadia Yosef ztz"l, examined the issue in a halachic responsum, concluding that there was no prohibition here whatsoever, but could be considered as a custom only. Here we see some of the unanswered questions of how we view the medical concepts of past generations; how, or if, they fit into the halachic process, as well as the need to distinguish custom from law. What I like about my dinner with the Yemenite rabbi, is that we each learned something. We saw that our practices were not always black and white, but represented shades of gray. Also, we remained friends, even better than before.