A question on a gemara - Shabbos 23a says the basis for the bracha of Chanukah is the lav of "lo sasur". The Sefer HaChinuch lists the beginning of the pasuk ("Al pi HaTorah asher yorucha...") as the Aseh to follow the Rabanan, and the next mitzva of "lo sasur" as the lav.
(a) Why does the Gemara chose the second half of the Pasuk for the basis of the mitvas asseh (miderabanan) - which is a lav - and skip over the beginning of the pasuk which is the actual asseh?
(b) Further, the Rambam in Mamrim 1:2 indeed does list the beginning of the pasuk as the source (which brings up the question why the Rambam did not use the Gemara's source, but that is another issue).
(c) Further, the Torah Temima also brings a Rosh in Kesuvos that says we don't have anywhere a birchas mitzva for fulfilling a lav, so why would Chazal be mitaken a bracha m'dirabanan (Chanukah) based on the lav, instead of the simpler approach which is the aseh "al pi haTorah"?
If the Rav can shed any light on the sugya, I would be very appreciative.
Kol tuv,
Ari
(a) The Gaon Rav Elchanan Wasserman asks your question in Koveitz Shiurim (vol. 2 notes on Shev Shma'atza Os Tes). Unfortunately, he does not answer! We could ask the same question of the Gemara in Berachos 19b which says that all d'Rabanans are based on the Lav of Lo Sasur. (The Margenisa Tava in fact asks this question (Shoresh Aleph Os Heh DH Amnam Kasha (p. 441 in Frankel edition)) but his answer does not help us, and is rejected by Rav Elchanan on the grounds that it is contradicted by the Rambam himself.)
Perhaps the Gemara prefers the Lav since it is Chamur as it is a Lav that has Misas Beis Din. Alternatively, it is possible that with regards to the Pasuk of "Al Pi ha'Torah" one may argue that this implies only to listen to Beis Din's interpretation of the Torah but not to their Takanos as the Ramban explains. The Lav lends itself more to the understanding that we must listen also to their Takanos. (The Ramban in fact explains that the Gemara means an Asmachta from the Pasuk.) Tosfos say in many places that the Gemara prefers the obvious Derasha. See also Tsapichas bi'Dvash (37) who seems to be saying that since there is both a Lav and an Asei the Gemara wants to tell us that he is even obligated due to the Lav.
(b) I am unsure what you mean to ask on the Rambam in Mamrim. He brings the Asei (1:1) and the Lav (1:2) and is not referring to Berachos at all. I presume you meant to ask from Rambam Berachos (11:3). If my answers are correct this is also explained.
(c) The Beracha is not for the Lav. The Beracha is for the Mitzvah d'Rabanan; the Lav is what obligates us to keep the Mitzvah d'Rabanan.
Dov Freedman