Shalom Rabbi. I have question about story of R. Chanina. What is meaning of this " baraita: If a Sage ruled an item impure, his colleague is not permitted to rule it pure; if he prohibited it, his colleague may not permit it?" Does this mean any time a rabbi prohibits/permits, another rabbi cannot go against decision?

melech yacov, houston

The Gemara is not implying that one cannot dispute the decision of the other sage. The Gemara is discussing when one sage has already ruled on the matter, can another sage rule in accordance with his dissenting view. This would belittle the honor of the first person and therefore it is proscribed to go from rabbi to rabbi after one has already received a decision. (Rashi Nidah 20b) Moreover it would make the Torah look like two different Torahs.

There is also an opinion that once a sage has ruled it actually becomes prohibited due to his psak and the other sage may not permit it afterwards. (Ritva Avoda Zarah 7a). However the poskim state that this is only if they were equal in stature but if the second one is more prominent he may permit what the first one forbade. This principle only applies to a particular case but not to other similar cases which come up. (Sheyarei Knesset Hagdolah Yoreh Deah 242). Others state that this only applies if the original psak was based on an erroneous assumption but if it was a mistake in a halakhic concept which is well known (Dvar Mishna) the second sage can rule differently.

Yoel Domb