“…there are two ‘Torahs’ – the Written and the Oral… both were given to Moses at Mt. Sinai and during the forty years in the desert, and taught to the whole nation. [They] have been with us… for the past 3300 years. And without both, it is impossible to fully understand traditional Jewish teaching or thought. The Written Torah, mentioned each of the Commandments, or Mitzvos, only in passing or by allusion. The Oral Law fills in the gaps… [it is like] an instruction manual.
Here is an example: ‘And you shall tie them as a sign on your arm and for (Totafos) between your eyes.’ (Deut. 6,8) This is the source for the Mitzvah of Tefillin… but it doesn’t tell us that much. From this alone, we’d never know how to do this Mitzvah. What are we supposed to tie to the arm? With what do we tie it? What are ‘Totafos?’ What is it a sign of? Without the Oral Law, quite simply, there’s no Mitzvah of Tefillin.
…So if it’s an ‘Oral’ Law, [why is it] written down?
In the lifetime of Rabbi Yehuda Hanasi (around 1700 years ago), Roman persecution, the recent destruction of the second Temple and the disruption of stable Jewish community life threatened our ability to properly retain and transmit this oral law. Rabbi Yehuda, therefore, wrote down the bare basics in the Mishna. A couple of centuries of hardship and persecution later, the rabbis of Babylonia saw a need to record even more detail and compiled a written version of what is known as the Talmud…”