The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
8. And, again, (the pradhâna cannot be active) because the relation of principal (and subordinate matter) is impossible (between the three gunas).
For the following reason also activity on the part of the pradhâna is not possible.--The condition of the pradhâna
consists in the three gunas, viz. goodness, passion, and darkness, abiding in themselves in a state of equipoise without standing to one another in the relation of mutual superiority or inferiority. In that state the gunas cannot possibly enter into the relation of mutual subserviency because thereby they would forfeit their essential characteristic, viz. absolute independence. And as there exists no extraneous principle to stir up the gunas, the production of the great principle and the other effects--which would acquire for its operative cause a non-balanced state of the gunas--is impossible.