The gemara gives two mekoros for the idea that a takanah or tzivuy cannot easily be changed or disregarded. One from the pasuk, for doraysa. The other, for takanos dirabanan, is a maaseh from R. Eliezer by Kerem Rivai. I am wondering, how can the gemara argue that a takanas chachamim cannot easily be changed from a maaseh invilving the chachamim? It seems that the Torah only considers that Dioraysahs need a minyan, but Drabanans do not. It seems that the rabanan themselves applied the idea of ain davar shebiminyan..to takanos dirabanan.
J. Z. Siegel
Thanks for your question. Your assumption is definitely correct. The Torah requirement for a Minyan Acher, only applies to Torah prohibitions. When Chachamim made a Takanah, they had the option of applying the same rule to their Takanah, or to decide that their Takanos can be overridden in an easier manner. For this we bring two sources, the first shows us that the rule is a Torah rule. The second source with R. Eliezer shows us that the requirement applies even with regard to Takanos Chachamim. What you say however is correct, they sure could have declared otherwise with regard to their Takanos.