The essence of mind is somewhat difficult to explain, so we look at it from the negative point of view, that is, what mind is not. First of all, we see that it is not something which arises or ceases or abides. It is free of these three things. From beginningless time, there is no arising, no cessation and no abiding in terms of staying in one place, not moving, or not changing. It is completely free of all three of these.
It is also free of being a thing or a substance composed of particles. The essential entity, or substance, of mind is not something that can be defiled or stained by grasping at subject and object. It is completely free of the stains from those activities.
Further, when we look at the essential substance of mind, we find that no matter how much we search for it, no matter how much we analyze it, there is no thing there to be found. There is no entity that we can come up with by searching, evaluating, and analyzing. No matter how much we seek for its essential substance, we cannot find it. The searcher, the one who does the search for essential substance of mind, cannot find it. Therefore it is said that the essential substance of mind itself is emptiness.
From The Practice of Mahamudra by Drikung Kyabgon Chetsang Rinpoche, page 48