Don't Add

The possuk in Vaeschanan says (4:2) לֹ֣א תֹסִ֗פוּ עַל־הַדָּבָר֙ אֲשֶׁ֤ר אָנֹכִי֙ מְצַוֶּ֣ה אֶתְכֶ֔ם וְלֹ֥א תִגְרְע֖וּ מִמֶּ֑נּוּ.  A similar verse appears in our parsha (13:1) .לֹא־תֹסֵ֣ף עָלָ֔יו וְלֹ֥א תִגְרַ֖ע מִמֶּֽנּו.  The Gra says that the first possuk is referring to beis din not to add onto the mitzvot, hence it’s said in the plural tense, referring to the collective beis din.  The possuk in our parsha is talking to the individual not to add to the specific mitzvah that he does.  (See the Ktav V’kabbalah in both places.)

This would be the source for the Rishonim that assume that it’s a prohibition to add to the number of the mitzvot (see Ramban on the possuk here, Rambam Ch. 2 of Mamrim etc.)  The Gemorah only assumes בל תוסיף in the context of adding to a mitzvoh, we don’t see that adding to the number of mitzvot is בל תוסיף?  According to the Gra the source is the possuk in Vaseschanan.  [Though it is noteworthy that the Rambam reverses the pessukim, in Birchat Kohanim (14:12) regarding adding to a mitzvah, he cites the verse in Vaeschanan, in Mamrim he cites the verse in our parsha.  See more in Minchas Asher Vaeschanan.]   

The Rashba in Rosh Hashana (16a) asks when Chazal said not to blow shofar on Rosh Hashana that falls out on Shabbos, why is it not a violation of בל תגרע?  The Turay Even questions that seemingly according to the Rashba , every time one doesn't fulfill a positive commandment, it will be a violation of בל תגרע as well?  The Baruch Taam and Rav Peurlo (on Rav Saisai Gaon pg. 81b) explain that the Rashba doesn't mean anyone who doesn't fulfill a commandment it will be a violation of בל תגרע; he is questioning that Beis Din should be violating the issur by cancelling out the mitzvah.

The Raavad (Mamrim 2:9) says א''א כל אלה ישא רוח שכל דבר שגזרו עליו ואסרוהו לסייג ולמשמרת של תורה אין בו משום לא תוסיף אפילו קבעוהו לדורות ועשאוהו כשל תורה וסמכוהו למקרא כדאשכחן בכמה דוכתי מדרבנן וקרא אסמכתא בעלמא ואם גרע לפי צורך שעה כגון אליהו בהר הכרמל אף זה דבר תורה הוא עת לעשות לה' הפרו תורתך. ולא תמצא איסור מוסיף אלא במצות עשה כגון לולב ותפילין וציצית וכיוצא בהן בין לשעה בין לדורות בין שקבעה בדבר תורה בין שלא קבעה:  Many understand the Raavad to mean that he disagrees with this additional principle of the Rambam and holds בל תוסיף is only when an individual adds to a mitzvah.  The Briskor Rav said that's not the peshat.  The words of the Raavad, בין לשעה בין לדורות בין שקבעה בדבר תורה can only be referring to Beis Din adding a mitzvah.  What he disagrees is that he holds the issur is only when adding a positive  command, not when adding a negative command (like the Rambam talks about, adding to the issur of milk and meat.)  However, the Rambam holds the issur is any addition. (Yeshurin volume 11 pg. 494.)

Of course, Rashi doesn’t learn this way and holds both pessukim are an issur not to add to a mitzvah being done.  What is noteworthy, is the differences in the Rashi.  In Vaeschanan, he says כגון: חמש פרשיות בתפילין, חמשת מינין בלולב, חמש ציציות.  In our parsha, he says חמש טוטפות, חמשה מינין בלולב, ארבע ברכות לברכת כהנים.  Why switch the third ex. from tzitzit to birchas kohanim and why switch terminology from פרשיות  to טוטפות?  You can find answers yourself.