The concept of Ba’al Tosif appears in Devarim (13:1) where the Torah states:
אֵת כָּל-הַדָּבָר, אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם--אֹתוֹ תִשְׁמְרוּ, לַעֲשׂוֹת: לֹא-תֹסֵף עָלָיו, וְלֹא תִגְרַע מִמֶּנּוּ.
All these law that I am commanding you to guard and do, you shall not add to them nor shall you diminish from them.
There is a dispute as to the precise understanding of this verse. The Gemara deals with the problem of sitting in the Sukkah on the day after Sukkot. The person sitting may be doing it only for their enjoyment, but it appears like he is adding details to the existing Mitzvah, and as such one should not sit in the Sukkah in Israel the day after Sukkah.
This approach is the one that appears in Rashi’s commentary on the above verse:
You shall not add: five compartments in tefillin, five species in the lulav, four blessings in the priestly blessings.
Rashi’s understanding of the verse is consistent with the above view of adding details to Mitzvot that were not outlined in the Torah. The three examples given are not examples of things that would be normally done by someone casually. Sitting in the Sukkah may be natural on a warm autumn day, but taking the lulav is not natural, and no one would take a lulav if not for Sukkot, hence adding details to that Mitzvah clearly fall under the category of Ba’al Tosif.
Sforno commenting on the same verse seems to imply that the problem of adding is directly connected to adding an entire Mitzvah that did not exist previously. This does not seem to be the simple understanding of the later sources.
The Mishnah in Zevachim (8:11) discusses the concept in further detail and seems to fall on the side of Rashi in this dispute.
The whole chapter deals with different parts of different sacrifices that were accidentally mixed and how we respond to it. The Mishnah in question deals with blood that should be sprinkled four times around the mizbeach getting mixed with blood that should only be sprinkled once. There is no doubt that the blood can be sprinkled and does not have to be disposed of, since both blood samples should be thrown on the same place, however how many times should it be sprinkled? Once or four?1
Rabbi Yehoshua says that there should only be one sprinkling. Rabbi Eliezer responds that there should be four sprinklings since with only one sprinkling we would be diminishing the four sprinklings and the three extras are ignored. Rabbi Yehoshua counters this argument by saying that Rabbi Eliezer’s suggestion entails adding to the Mitzvah.
The argument then continues that the laws of Baal Tosif are only applicable when there is no doubt and the blood is not mixed, however now that the blood has been mixed and one of the concepts (adding or diminishing) has to take precedence and the Mitzvah of Baal Tosif is waived. The only question that remains is should we work to a minimum or a maximum, that is should we get all four sprinklings in despite the fact that there is some blood that requires only one, or should we do only one sprinkling and not perform the three excess sprinklings for the sacrifice that requires four?
Rav Yehoshua then continues his argument:
כשנתת--עברת על "בל תוסף", ועשית מעשה בידך; וכשלא נתת--עברת על "בל תגרע", ולא עשית בידך.
When you transgressed adding to a Mitzvah you did so actively (by doing the extra sprinklings), whereas when you transgressed diminishing a Mitzvah you did so, but without doing an action (since all that was done was not sprinkling blood).
It is this argument that finally determines the Halacha, and we only throw the blood on the mizbeach once.
There are however two significant lessons that can be derived from this dispute. The first is the concept of Ba’al Tosif is only applicable in a situation where the case is clear cut and no other possibility legitimately presents itself. These are the cases that Rashi presented in his commentary, however he deliberately avoided using the case of Succa, since as we have described above, there is a possible alternative explanation.
The second principle derived from the Mishnah is one that appears numerous times throughout halacha, which is that when in doubt it is best to not do anything rather then do something that may be questionable.
1 Editor’s note: “Blood that should be sprinkled four times”, should not be understood as referring to the blood of a chatat offering that is quite literally place on the four corners on the top half of the mizbeach but rather referring to those sacrifices that require two sprinkling which are placed on the corners of the mizbeach making them really four – for example an olah or asham offering. This is because such blood is placed on the lower part of the mizbeach like those korbanot that require one sprinkling.
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