Ask many Orthodox and Conservative Jews about the origin of Judaism’s oral law and they will tell you that God revealed both the written and oral Torah to the Israelites through Moses at Sinai. Actually, the Bible only states that the Ten Commandments were revealed at Sinai. The rest of the Torah, with incidences that occurred after the Sinai revelation, was given at a later time in different places. Furthermore, while many Jews believe otherwise, the interpretation of the Torah laws by the rabbis, called the oral law, developed long afterwards, when Jews recognized the need to modify the Torah teachings. Rabbi Binyamin Lau, an Orthodox rabbi, discusses the development of the oral law during the second temple period in his book The Sages.
For example, the Torah mandates in Deuteronomy 15:2 that “every creditor shall release that which he lent to his neighbor” during the Shemitah year, every seventh year. Hillel, who lived at the beginning of the Common Era, changed the biblical law and allowed creditors to keep the loan under certain conditions. This change was accepted by the Jews and became part of the oral law. A more radical example is that the Torah contains 36 crimes punishable by death, but the oral Torah imposed so many conditions that it made the death penalty virtually impossible.
Maimonides recognized this in his Guide of the Perplexed 3:41. He states that the Torah requires a punishment of a limb for a limb, in Leviticus 24:20 and other places, but current law imposes only compensation. He writes that he won’t explain the reason for the change: “For we proposed to ourselves to give here the reasons for the precepts mentioned in the Law, and not for that (the changes) which is stated in the Talmud,” apparently because even in his day the concept existed that the oral law was revealed at Sinai, and he didn’t want to disturb the people.
It is true that Maimonides lists 613 “biblical commands,” in his Book of Commandments and Mishnah Torah, and these include the modifications introduced by the oral law. However, readers shouldn’t suppose that he was recording what he thought were divine commands; he was listing what the rabbis considered divine commands, that is the Torah laws as interpreted by the changes of the oral law.
How then should Jews behave? Yeshayahu Leibowitz wrote in his Accepting the Yoke of Heaven that virtually all the Torah laws were altered by the rabbis in what is called the oral law, and Jews today are not Torah Jews, but Jews who follow the Torah as interpreted by the rabbis.
This phenomenon is not unique to Judaism. It exists for all religions because conditions change, and laws must change to accommodate them.