Without any duality of perceiver and perceived, there is no way a normal thought can survive; it vanishes.
The phrase: ‘single sphere of dharmakaya’ refers simply to this original wakefulness. It is called single or sole, meaning not a duality, whereas the normal thinking mind is dualistic, and is never called single.
If this holding onto duality is not dissolved from within, there is the perpetuation of subject and object, perceiver and perceived.
Another famous phrase goes: ‘As long as duality does not become oneness, there is no enlightenment.’ When recognising, this duality is dissolved into oneness.
Tulku Urgyen Rinpoche