'The Jewish tradition is full and complete, not because it relies only on an ancient single sources, the Bible, but because it is open to additions.
All the accumulated oral traditions are considered part of the original written Torah.
Even details of the oral Torah, obviously belonging to a much later period, are considered to be continuations of the original revelation.
It is all the same revelation, written or oral, and includes the ancient text and the ever-changing unwritten social form and custom."
From Parabola Magazine 14:2, pp. 95-102