(Treasure Store Treatise)

Within heaven and earth, inside all the cosmos, there is contained a singular treasure concealed in the form-mountain—the numinous radiance of sentient things. Utterly empty, still, and difficult to perceive within or without, it is styled the “mystery of mysteries.” Its skill reaches out beyond the [celestial palace] of Purple Subtlety, and its function resides in the very midst of empty non-being. Unmoving among manifold transformations, it is solitary and nondual. Its voice brings forth wondrous reverberations; its form spews forth iridescent displays. But look as you will, it has no locus; it is known to us as the emptiness of emptiness.

僧肇 –Sengzhao, Pao-tsang lun
(Treasure Store Treatise)