Moses was given an Oral Law together with the Written Law.
The Oral Law included many things that were necessary for
effective understanding of the Written Law, for example
details about honoring parents, observing the Sabbath, etc.
It also included some things that are independent of the
Written Law, such as some of the observances in the Temple
on Tabernacles (the water libation and the procession around
the altar with willows), but such things seem to be rare.
Thus most of the Oral Law explains and supplements the
The Oral Law was of the same Divine origin as the Written Law.
But after the Written and Oral Laws were given to Moses, oral
laws continued to be added by the Rabbis, beginning with Moses
himself; thus the Divine part of the Oral Law was immediately
succeeded by parts that were of human origin (though they too
were Divinely inspired). A compilation of the oral laws, the
Mishnah, was codified by Rabbi Judah the Prince, head of the
Sanhedrin, about 150 years after the destruction of the Second
Temple, i.e. about 1800 years ago; but it has continued to grow
since that time. One of the most comprehensive codes is
Maimonides' Mishneh Torah, written around 1200 C.E.
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