"Non-observing is bodhi because it transcends all conditions;
Non-mentation is bodhi because it is without mindfulness;"
Excerpts from A Discourse on the Vimalakīrti Nirdesa Sutra
Venerable Xiao Pingshi
The notion of “observing” here also includes thoughtless pristine awareness of our conscious mind, which never stops observing the six sense objects. Regardless of the level of concentration skill we have attained or whether we have entered samāpatti (a deep state of meditative absorption) of the second dhyana or have not, we will still have mental objects in the samadhi state. As we need to observe the state of our meditative absorption to decide whether to remain in such state or not to, our conscious mind with pristine awareness will constantly engage in observation, although such observation will no longer involve observing the five sense objects.
The tathagatagarbha, on the other hand, never observes any state of any of the six sense objects and does not correspond to the meditative state of the independently arising mental consciousness existing above the second dhyana because it denotes absolute quiescence and extinction. That is, the tathagatagarbha does not need to observe or contemplate because it is inherently quiescent and extinct. It is “non-observing,” not observing any state whatsoever. This denotes the attainment of true bodhi as the tathagatagarbha transcends all conditions, perceptions, and differentiation of the six sense objects and does not observe any state of any of such objects.
Our minds are constantly busy with mental activities. Mentation is the process by which our functional mind continuously discerns the six sense objects. A mind in a constant flux of activities cannot be the true bodhi mind. In view of this, a functional working mind taking in the six sense objects would give rise to illusive thoughts. Why? Our minds tend to be attached to our recollections of various objects. While we are engaging in sitting meditation, thoughts might crop up in our minds due to the joint operations of our mental faculty and our conscious mind. Recollections belong to the five specific mental concomitants and are not the Dharma wisdom. The recollections of the conscious mind are brought about by a mental function that works together with the six sense objects. The True Mind, tathagatagarbha, neither gives rise to any mental activity of any of the six sense objects nor is mindful of any dharma. This is what the verse “non-mentation is bodhi because it is without mindfulness” means.
A Discourse on the Vimalakīrti Nirdesa Sutra, Vol. 2, p.249-250