Dov Lew asked:
The Sefer Yeraim [near the beginning of Siman 274] writes that the license to choose what should be included within the category of Avos
Melachos was granted to the Chachamim. Isn't this contrary to our Braissa that all of the Avos Melachos were given to Moshe on Sinai?Dov Lew
The Kollel replies:
The Beraisa says that only the number of Melachos was given to Moshe Rabeinu at Sinai. The exact identity of each of those 39 Melachos was not explicitly stated. The Torah gave the license to the Chachamim to clarify what those Melachos are.
(This is similar to the prohibition of work on Chol ha'Mo'ed. The prohibition itself is d'Oraisa, according to some Rishonim, while the actual labors that are prohibited was left in the hands of the Chachamim to decide. See Chagigah 18a.)
Kol Tuv!