Daf Yomi - Yoma 80 - Halachos that were Forgotten

The Gemora brings an argument regarding שיעורין. One Tanna holds that all the שיעורין are learned from a הלכה למשה מסיני and others hold that the court of Asniel ben Knaz established them and later they were forgotten and reestablished.

The Rambam states that it is impossible to argue on a halacha l'Moshe misinai and there cannot be arguments regarding that halacha. The Brisker Rov explains our Gemora. The first Tanna held that the halacha of 'amounts' were never forgotten and therefore there is no court that can ever change or argue on this. there is never a possibility that a future court will say that one will not be liable unless a larger שיעור is eaten. The second opinion held that this halacho was from the three thousand halachos that were forgotten in the time they were mourning for Moshe. The beis din of Asniel was able to reestablish these halachos using the thirteen principles that the Torah can be expounded with. Therefore it is possible that a future beis din can come and disagree or change the size of certain שיעורין.