"The mind is empty, cognizant, united, unformed. Please make the meanings of these words something that points at your own experience. You can also say the mind is the “unformed unity of empty cognizance.” These are very precious and profound words. “Empty” means that essentially this mind is something that is empty. This is easy to agree on: we cannot find it as a thing. It is not made empty by anyone, including by us - it is just naturally empty, originally so.
At the same time, we also have the ability to know, to cognize, which is also something natural and unmade. These two qualities, being empty and cognizant, are not separate entities. They are an indivisible unity. This unity itself is also not something that is made by anyone. It is not a unity of empty cognizance that at some point arose, remains for a while and later will perish. Being unformed, it does not arise, does not dwell, and does not cease. It is not made in time. It is not a material substance. Anything that exists in time or substance is an object of thought. This unformed unity of empty cognizance is not made of thought; it is not an object of thought."
~ Chokyi Nyima Rinpoche