Why is the manas the ruling consciousness?

The manas is the consciousness that acts as the master. Among the five aggregates and eighteen realms, all matters great and small are directed and commanded by the manas as the master. No one can substitute for the manas—not even the mind consciousness. Although the thinking and judgment of the mind consciousness can guide the manas's decisions, it is ultimately the manas that makes the final decision as the master. The tathāgatagarbha also cannot make decisions in place of the manas, cannot direct the bodily, verbal, and mental actions of the six consciousnesses, and cannot act as the master in worldly dharmas.

Because the tathāgatagarbha is without a self-nature and does not see any worldly dharmas—what He sees are all dharmas of a non-worldly nature—He cannot act as the master over dharmas of a worldly nature, nor can He direct the creation of worldly dharmas. He does not have the mental conduct of "I" like the manas does; without a notion of "I," He will not act as the master.

Cultivation is precisely the removal of the manas's ignorance, letting Him know that the five aggregates and eighteen realms are not the self, knowing that the true master is the tathāgatagarbha, and that all dharmas are false appearances manifested by the tathāgatagarbha, not belonging to the manas itself. Only in this way can the manas gradually eradicate the grasping of self and the grasping of dharmas; when all grasping is exhausted, one becomes a Buddha.