Matters of doubt[1]
על-פי התורה אשר יורוך ועל-המשפט אשר-יאמרו לך תעשה לא תסור מן-הדבר אשר-יגידו לך ימין ושמאל
You shall do according to the Torah that they rule for you, and the judgement that they tell you. Do not turn left or right from the matter that they tell you[2]
The Rambam, also known as Maimonidies, learns from this verse[3] the obligation to listen to the Rabbis. It comes out then that every Rabbinic mitzvah, obligation, or prohibition, are all included in the commanded not to turn from the matter that they tell you. That should make them all obligatory on a Biblical level in some way. To this asks[4] the Ramban, also known as Nachmanidies, how could it be then that we have a rule in a Biblical matter of doubt that one must be stringent, but in a Rabbinic matter of doubt one may be lenient? If every Rabbinic matter is really Biblical, how could there be this distinction?