Woe to those who separate the sign of the covenant, ‘between waters and waters’, and prevent seed from it, for he returns the world to chaos and void. And the mystery of the matter: (Is. 45:18)... not for chaos did He create it, for dwelling did He form it... specifically.
The ‘higher waters’ are the Written Torah, the lower waters are the Oral Torah. The ‘hair’ which is between both-of-them – this is Yesod, which is ‘the mystery of the Torah’, and it is the foundation and essential principle of both-of-them.