The Mishnah in Pesachim 39a lists five types of vegetables that can be used for marror. Where did the Sages derive this list from? Is it some kind of Halachah L'Moshe MiSinai, or Chazal figured out that these species best fit the description of מרורים stated in the Torah, or some other reasoning?
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I can't quite answer your question, but this could get you started:
A. Depending on how you translate the items listed in the Mishna and Gemara there, they may all be members of the composite/lettuce/daisy family. (The one that stands out like a sore thumb is horseradish, which Rabbi Hershel Welcher admitted does not fit according to modern-day analysis of the Gemara; he uses mostly romaine, with a touch of horseradish for nostalgia's sake. The Talmudic "tamcha" came to mean "horseradish" in Hebrew by the 1500s, but it's far less than clear that's what the Talmud was referring to.) So there was likely a tradition "anything in this family with these properties." It may really have been one tradition, not five separate ones.
B. See this mp3 from Rabbi Aharon Kahn. The Gemara talks about how to identify the Four Species (lulav etc.); once it's narrowed it down to one or two, it disqualifies one of them as it has sharp barbs that would injure your hand when holding it; if there are two equally reasonable interpretations, we assume, based on the principle of "the Torah's ways are pleasant", that it's the less-painful one. Now one could look at that whole Gemara and ask to what degree Chazal are simply validating their tradition of the identity of the Four Species, or if this is the derivation. I don't know the subject well; but it may be related to the question of how halachic "derashot" work, on which some Gaonim held it's entirely tradition, and Rambam held it's entirely derivation. Some ask that a barbed plant was not disqualified from the marror list; Chasam Sofer answers brilliantly that you have a choice of bitter herbs. If the Torah is obligating you to take a kapot temarim, it stands the reason the Torah isn't demanding you tear your skin (unless we're told otherwise). But if the Torah says "eat any of these plants; most of them are not scratchy", and you want to choose to make yourself miserable and do the scratchy one? That's your prerogative!