Don't Do The Mitzvah

The Gemorah in Yevamot (90) says the Chochamim have the ability to uproot a mitzvah from the Torah by telling one to passively not do the mitzvah.  The Teshuvot Rashba (volume 6 #254) says this is based on the possuk of ככל אשר יורוך.  Rav Elchonon (Kovetz Ha'arot siman 69) has a chakirah if the Chachamim uprooted the mitzvah because of their takkanah or that the mitzvah remains in place but they said not to fulfill the mitzvah.  One of the נ"מ is when a kid is blowing shofar on Rosh Hashana Rabbi Akiva Eger (Derush V'chiddush maarachah 8) says one does not have to stop the kid for he is fulfilling a mitzvah.  He holds the mitzvah still remains.  R.A.E. goes lishitaso in Orach Chayim siman 128 on the Magen Avraham 4 he says that a kohan that is rabbinicly disqualified from doing ברכת כהנים should walk out before they are called up so as not to violate the mitzvah.  He clearly holds the mitzvah remains in place.  R.A.E. learns this is the peshat in Tosfos Sukkah (2a) questions why rain is considered a curse on Sukkot if one could just make schach that will not allow rain to come through?  Tosfos qualifies that even if such schach would be invalid rabbinicly that is not reason for rain to be considered a curse.  R.A.E. explains the reason is since if one violated the rabbinic decree and made schach in such a manner it would still be valid.  We see that Tosfos holds the Chachamam did not remove the fulfillment of the mitzvah because of their takkanah.  However, Tosfos Sukkah (3a) says that if one had their table sticking outside the sukkah which is rabbinicly forbidden according to Beis Shamai lest one follow the table outside the sukkah one does not even fulfill the Torah law, the Chachamim removed the fulfillment of the mitzvah?  How would this fit with the Tosfos on the previous page according to R.A.E.'s understanding? 

There are different forms of the takkanot of the Rabbanan.  It is only when the Chachamim gave a form to the maaseh mitzvah that if one doesn't adhere to the form they prescribed then they removed the kium mitzvah.  So if one doesn't sit in a sukkah in the way the Chachamim defined yeshivat sukkkah then one does not fulfill their obligation.  Similarly, Rabbenu Yona writes in the beginning of Berachot that one who says krias shema after chatzot of the night does not fulfill their obligation for the Chachamim defined the mitzvah of krias shema as being until chatzot.  However, when it comes to a takkanah that does not define the kium hamitzvah but is an external takkanah, then they did not rub out the mitzvah.  So if one blows shofar on Shabbat the Chachamam are not defining the form of blowing shofar, they made a gezarah not to do it on Shabbat.  Similarly, when it comes to a thick schach the action of the mitzvah is not being defined, the takkanah is defining a sukkah.  The Chachamam do not declare a sukkah not to be a sukkah.  It is only when the takkanah gives a presice definition to the maaseh mitzvah that the Chachamam revoked one's fulfillment it the mitzvah was not done properly.

It could be this chakirah is the machlokes Rabbenu Yona and the Rosh in the approach of the Pri Migadim.  Why does Rabbenu Yona disagree with the Rosh that says the Rabbanan said you are not yotzei?  That principle is only when the Rabbanan gave a form to the fulfillment of the mitzvah.  The Rosh holds kiddush not done במקום סעודה is a problem with the maaseh kiddush, the Rabbanan gave a formula how to do the mitzvah, if its not followed, you are not yotzei.  Rabbenu Yona holds like the Rivash, the Rabbanan didn't make a takkanah in how to do the mitzvah but added a detail to enhance the kium mitzvah. 

The Rosh in Arvey Pesachim siman 5 brings Rabbenu Yonah holds that even if there is no one is shul that is eating there, one can still recite kiddush in shul even though it is not in place of a seudah for the requirement of seudah is merely rabbinic and it can be ignored in place of keeping the custom to say kiddush in shul.  The Rosh disagrees because kiddush not in place of the seudah is not kiddush at all.  What does he mean?  The Pri Migadim (269:1 on Taz) says the Rosh means that once the Chachamim enacted the law of kiddush must be done in place of a seudah, if that is not kept, they revoked one's fulfillment of kiddush.  Why would Rabbenu Yona disagree if he holds of such a concept regarding the time of krias shema?  In light of what we said previously we can understand they argue as to what the takkanah of kiddush in place of seudah is.  The Rosh holds the Chachamim gave a form to the mitzvah like krias shema.  Rabbenu Yona holds the takkanah is not in the maaseh iddushin, to be magdir the maaseh kiddushin, it is an external takkanah to enhance the kium mitzvah of kiddush it should be done in place of a seudah (see Rivash #391.)  However, since the Chachamim were not formulating a din in the maaseh kiddush itself they did not remove the kium of kiddush if it was done not in the placeo f a seudah.  (This last paragraph is based upon a hesber given by Rabbi Elefant.)