The Oral Law
The Written Law is another name for the Torah. The Oral Law is a legal commentary on the Torah, explaining how its commandments are to be carried out. Common sense suggests that some sort of oral tradition was always needed to accompany the Written Law, because the Torah alone, even with its 613 commandments, is an insufficient guide to Jewish life. For example, the fourth of the Ten Commandments, ordains, "Remember the Sabbath day to make it holy" (Exodus 20:8). From the Sabbath's inclusion in the Ten Commandments, it is clear that the Torah regards it as an important holiday. Yet when one looks for the specific biblical laws regulating how to observe the day, one finds only injunctions against lighting a fire, going away from one's dwelling, cutting down a tree, plowing and harvesting. Would merely refraining from these few activities fulfill the biblical command to make the Sabbath holy? Indeed, the Sabbath rituals that are most commonly associated with holiness-lighting of candles, reciting the kiddush, and the reading of the weekly Torah portion are found not in the Torah, but in the Oral Law.
The Torah also is silent on many important subjects. We take it for granted that the large majority of couples want their wedding ceremony to be religious, but the Torah itself has nothing to say concerning a marriage ceremony. To be sure, the Torah presumes that people will get married — "Therefore shall a man leave his mother and father and cleave to his wife and they shall be one flesh" (Genesis 2:24) — but nowhere in the Torah is a marriage ceremony recorded. Only in the Oral Law do we find details on how to perform a Jewish wedding.
Without an oral tradition, some of the Torah's laws would be incomprehensible. In the Shema's first paragraph, the Bible instructs: "And these words which I command you this day shall be upon your heart. And you shall teach them diligently to your children, and you shall talk of them when you sit in your house, when you walk on the road, when you lie down and when you rise up. And you shall bind them for a sign upon your hand, and they shall be for frontlets between your eyes" (see Deuteronomy 6:48).
"Bind them for a sign upon your hand," the last verse instructs. Bind what? The Torah doesn't say. "And they shall be for frontlets between your eyes." What are frontlets? The Hebrew word for frontlets, totafot is used three times in the Torah — always in this context (Exodus 13:16; Deuteronomy 6:8, 11:18) — and is as obscure as is the English. Only in the Oral Law do we learn that what a Jewish male should bind upon his hand and between his eyes are tefillin (phylacteries).
Finally, an Oral Law was needed to mitigate certain categorical Torah laws that would have caused grave problems if carried out literally. The Written Law, for example, demands an "eye for an eye" (Exodus 21:24). Did this imply that if one person accidentally blinded another, he should be blinded in return? That seems to be the Torah's wish. But the Oral Law explains that the verse must be understood as requiring monetary compensation: the value of an eye is what must be paid.
For these three reasons-the frequent lack of details in Torah legislation, the incomprehensibility of some terms in the Torah, and the objections to following some Torah laws literally — an Oral Law was always necessary.
Strangely enough, the Oral Law today is a written law, codified in the Mishna and Talmud. Orthodox Judaism believes that most of the oral traditions recorded in these books dates back to God's revelation to Moses on Mount Sinai. When God gave Moses the Torah, Orthodoxy teaches, He simultaneously provided him all the details found in the Oral Law. It is believed that Moses subsequently transmitted that Oral Law to his successor, Joshua, who transmitted it to his successor, in a chain that is still being carried on (Ethics of the Fathers 1:1).
Given this chain of authority, one might wonder why the Mishna and Talmud are filled with debates between rabbis; shouldn't they have all been recipients of the same, unambiguous tradition? Orthodox teachers respond that the debates came about either because students forgot some of the details transmitted by their teachers, or because the Oral Law lacks specific teachings on the issue being discussed.
While Conservative and Reform Judaism also believe that some kind of Oral Law was always necessary to make the Torah comprehensible and workable, they reject the belief that most of the Talmud dates back to Moses' time. They are more apt to see the Talmud and the Oral Law as an evolving system, in which successive generations of rabbis discussed and debated how to incorporate the Torah into their lives. Thus, they feel more free than the Orthodox to ignore, modify, or change the Oral Law.
The differing views of Orthodox and Conservative Judaism on both the antiquity and binding nature of the Oral Law are one of the major, perhaps the major, issues separating them.
http://www.us-israel.org/jsource/Judaism/Oral_Law.html