The teaching that there is a pure consciousness that must be cleared of adventitious defilements (such as those stored by the storehouse consciousness) is, however, something that can be found as far back as the teachings in the Pāli canon.

This mind, O monks, is luminous, but it is defiled by adventitious defilements. The uninstructed worldling does not understand this as it really is; therefore for him there is no mental development.

This mind, O monks, is luminous, and it is freed from adventitious defilements. The instructed noble disciple understands this as it really is, therefore for him there is mental development. (Nyanaponika & Bodhi, p. 36)

If this is indeed the case, and there is a pure consciousness in the depths of our being, then Buddhist practice is not about creating an awakened state of mind but of recovering or rediscovering the awakened state of mind that was there all along. This is what is taught in the Awakening of Faith in the Mahāyāna Treatise: “Grounded on the original enlightenment is non-enlightenment. And because of non-enlightenment, the process of actualization of enlightenment can be spoken of.” (Hakeda, p. 38) How the pure consciousness came to be obscured by adventitious defilements in the first place seems to be an unanswerable question. The point of the teaching is that a pure awakened mind is always present and that our practice can wipe away the obscurations and allow it to shine like a bright mirror cleared of dust.