From the Commentary:
The Nature of the Mind is from the very beginning void or empty and without any self or concrete substance. But one should not think of mind as b eing a mere nothing because it has the clarity and the limpidity of a mirror. This clarity exists unobstructedly and without interruption, just as the moon may be reflected in the water in various ways. Thoughts arising in the mind are the way in which the Nature of Mind manifests itself. but just as one must understand the reflections in order to understand the nature of the mirror, so one must examine thoughts to see where they arise, where they abide, and where they go. However, when one looks into this matter, one discovers that there is no place where thoughts arise, abide, or go. Nothing can be affirmed and what one finds is void or emptiness. This is the real character of the mind. Now, even though this may be tghe case, thoughts continue to arisw without interruption. Therefore, what one finds ia a primal awareness of pure presence (rig-pa'i ye-shes), where there is no duality opf emptiness on the one hand and clarity on the other. This primal awareness is natural and spontaneously self-perfected. At the level of mind (sems), one does not find this non-duality because mind, or the thought process, opperates in time and conditioning, while the state of of pure presence or intrinsic awareness (rig-pa) lies beyond mind and its limitations.
[Excerpted from The Cycle of Day and Night, translated and edited by John Myrdhin Reynolds, Station Hill Press, Barrytown NY, 1987]