How can halachic restrictions that are not in the Torah be reconciled with the Deuteronomy 4:2 passage, which says, "Ye shall not add unto the word which I command you, neither shall ye diminish from it"?
Such halachic restrictions fall into two categories: those which we believe were given as part of the Oral Law along with the Torah, and those that were clearly put in place later by the rabbis.
The former is not adding, as it was part of the given word. E.g. Deuteronomy 12:21, to eat non-sacrificial meat, just slaughter it "in the manner I have prescribed"; the method of kosher slaughter does not actually appear in Tanach, it was transmitted orally until the Talmud codified it.
The latter make a strong point of being distinct from Biblical obligations.
Here's an excerpt from Maimonides' code, Hilchot Mamrim (laws regarding rebellion against rabbinic authority) Chapter 2:
As a rabbinic court can make decrees to prohibit that which had been [Biblically] allowed, and that prohibition stands for generation ... what is it that the Torah warned "thou shalt not add nor subtract?" Not to add nor subtract and establish something new forever and claim it as Biblical law, whether the Written Law or Oral Law.
How so? It is written in the Torah (Ex. 23:19) "thou shalt not cook a g'di in its mother's milk", and by oral tradition [of Biblical force] we learned that this verse prohibits cooking and eating the meat of any mammal, whether domesticated [such as sheep] or wild [such as deer], in milk; but chicken could be cooked and eaten with milk, as far as the Biblical law.
Should a rabbinic court go and allow wild-mammal meat in milk, they have "subtracted"; and if someone prohibits chicken in milk, claiming it was included in the Biblical prohibition on g'di, that would be "adding."
But if it was stated that: chicken [in milk] was permitted Biblically, but we [the rabbis] shall prohibit it, and inform the people that this is a decree to prevent problems; as people may say "chicken is okay because the Torah didn't literally say chicken, so it didn't say wild-animal either"; and the next person will say "any domesticated mammal is fine too except for goats", and the next person will say "goat meat is acceptable in the milk of cows or sheep, as it said its mother's, meaning that same species"; and the next person will say "I can cook a goat in goat's milk, as long as it wasn't this particular goat's mother" ... therefore, we shall prohibit all meat in milk, even chicken. This is not adding, it is creating a protective fence around the Torah. And so too all similar matters.
Excellent question. The answer is that the Rabbis themselves are invested by the Torah with the duty to protect the basic halacha - see for example Deuteronomy 17:8-11, where it basically states that we are Scripturally bound to comply with the rulings of the Sanhedrin. Furthermore see Pirkei Avos 1:1, which states, "... make a protective fence for the Torah." This means that the Rabbis saw it as their duty to make enactments to protect the central Scriptural commandments, in the same way that one would put up a protective cordon several feet away from an open manhole to make sure that nobody falls in.
Your question still is pertinent - is this not "adding" to the Torah? In order to avoid this problem, it is very important that we have to have a very clear distinction between those laws that are Scripturally mandated, and those that were enacted by the Rabbis. You will find several practical differences in our approach to Rabbinic mitzvos as opposed to Torah mitzvos; there are a great many leniencies in practical halacha regarding Rabbinic mitzvos.
There are basically 2 types of additions which are allowed:
Those that prevent us from accidentally transgressing - as it says in the first Mishna in Pirkei Avot: Make a fence around the Torah.
Those instituted by the Sages and Prophets under the umbrella of " כְּכֹל אֲשֶׁר יוֹרֽוּךָ" - "you shall do whatever they teach you" (Devarim 17:10).
Keep in mind that not everything is written in the Torah; the Oral Law has many details missing from the Written Law. E.g.: How to make Tefillin, how to slaughter properly and how many walls a Sukkah needs.
Many Halachic Restrictions were given to Moshe at Sinai as part of the Oral Law, along with the written law.
Ye shall not add unto the word which I command you, neither shall ye diminish from it, that ye may keep the commandments of the LORD your God which I command you.
Verse 2: Do not add. For instance to place five parshiyos in the tefillin, to take five species [of fruit] for the mitzvah of lulav, to place five fringes [instead of four on one's garment]; and this also applies to (the command), "And do not subtract from them."
The various commentaries point out that all of Rashi's examples are adding or subtracting from a commandment that has a specific number associated with it. So, for example, if you have a garment with 5 corners, don't put tzitzit strings on the 5th corner. Doing so would violate this commandment. However, As the Gur Aryeh says, the Rabbinically legislated prohibitions which augment the Biblical restrictions are not considered additions, since they are clearly intended as protective measures to guard against infractions of the Biblical commandments.
The Chumash Shai LaMora brings this in the name of the "Sefer Zichron".
There are at least four answers to this question: Instead of explaining them, I will provide the relevant citations.
- Maimonides which has already been cited here.
- R. Judah Halevi in the Kuzari 3:41.
- R. Joseph Albo, Sefer ha-Ikkarim 3:14, which is also the view of Maharal (Be'er ha-Golah, beginning).
- Maharsha to Megillah 14a, as well as R. Chaim Volozhiner, Nefesh ha-Chaim 1:22.
(For an in-depth discussion of Maimonides' view, see here).