Here is this related from simpletoremembers Jewish site:
http://www.simpletoremember.com/articles/a/oral-law-written-law/
1. What is the Oral Tradition (Torah SheBaal Peh)?Torah SheBaal Peh (the Oral Tradition) is oral because ultimately it has no repository other than the minds of the Jewish people. Even when it was written down in the form of the Talmud, it was written is such a way that it retains a component which can only be learned from a teacher. The Oral Law is the way in which we connect with the Torah. It is the bringing of the Torah into this world all the way down until it rests in and connects with our minds [1] . Without the Oral Law, the Torah would always be something separate from ourselves, as is the Written Torah. This would render the Torah something abstract and removed. But this was not G-d’s Will. G-d’s whole reason for creating the Torah was so that we could keep it – so that we could become walking Sifrei Torah. The Oral Law is therefore not a luxury. It is G-d’s way of ensuring the implementation of His plan [2] .
2. How do I know that it is true? I Can Accept The Written Torah But Not The Oral Torah.It is quite obvious from looking at the Written Law that it was intended to be accompanied by an oral component. There is not a single mitzvah in the Torah that can be understood in all its parameters without resort to the Oral Law. [3]
For example, the Torah states that the Tefillin should be as Totafos between the eyes. But there is absolutely no indication from the Torah as to what this word means. Are the Tefillin meant to be big, green round balls worn on our nose, or sharp red pointy ones worn on our foreheads. Maybe we should be wearing a smooth, flat, gold plate. Should there be anything written involved? What? And on what? And where should this be placed? The Written Torah does not give even a clue. Now if G-d has given us this Torah as a revelation of His Will, then he has played upon us a cruel joke. It is like the NY Subway announcements. We hear clearly the bit about, “And the next stop will be…” But just when we get to the crucial information, all we hear is static. It cannot be that G-d went to all the trouble to create this world for us, then gave us the Torah to tell us how to live here, and we open it up, it is incomprehensible. Clearly, there is another place, other than the Written Law where He tells us what Tefillin are and where we should wear them [4] .
G-d tells us not to work on the Sabbath, but nowhere does the written law define work. Nor does it tell us how many categories of work there are [5] . G-d tells us to slaughter an animal, but the Written Torah does not tell us how. Are we supposed to slaughter it from the neck or the throat. And with what kind of instrument? The Torah says that you should slaughter, “As I have commanded you [6] ”, implying that we know from somewhere else how to do this. But nowhere else in the written Law is anything clarified on this issue. The information must then be in the Oral Law. There are many, many such examples [7] .
There is a group of people called the Karaites who insisted that there is no such thing as the Oral Law. But, since the text did not tell them clearly what they had to do, they simply ended up inventing their own oral law. They chose man’s interpretations over G-d’s. A poor choice indeed.
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