Explanation:

The entity that does the work of knowing is the inert energy or Atman that remains in deep sleep. It is eternal because inert energy is never destroyed even in the final dissolution of the world. This Atman is nirguna, meaning devoid of qualities. Here the word quality is restricted only to the property of awareness and not a property in general. In deep sleep, awareness disappears and hence all its qualities also disappear. This inert energy which is left over in deep sleep, and which is devoid of qualities, is called as Nirguna Atman. Remember that the word quality here is confined only to the property of awareness and not to any inert property.