....If all is Brahman, then that must include both that which is seen and that which is the seer. ....
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That is my premise also. The Self -Brahman is neither the Seer or the Seen, as it is only the 'I' with ideas and knowledge that sees but Self is just Is.
But, in the mind level (phenomenal level ) the Seen is not ever at par with the Seer. There are several hierarchal Seer-Seen layers. The Seer and the Seen are identical (advaita) only in transcendental realm.
For example, in dream one Sees a dream body as 'me'. But that seen body is not at par with the Seer.
Drik Drisya Viveka of Shankaracharya explains about seer and seen.
When we see forms and colours through eye, the eye is the seer and forms and colours are seen.
However, eye cannot see the objects unless the objects seen by eye are recognized by mind. Then the mind becomes the seer and the eye becomes seen.
But, the mind cannot recognize the objects unless it gets its power from atma. This, we daily experience in our deep sleep where the mind is withdrawn and we are not aware of any objets.
Ultimately, Witness/ aatma/Self/I/god whatever name we call is the Seer (subject) and all others are Seen (objects) and hence inert only. However, the objects such as the physical body, the subtle body (consisting of mind etc.) and the causal body gets their sentiency due to its association with aatma.
The point is that, as long as there is subject-object division, it is the Seer that alone sees and the Seer itself cannot ever be seen. But Seer can know itself only by dissolving the Seer-seen dichotomy of its own mind when the mind that gives rise to subject-object division is involuted in Brahman and becomes Brahman itself.
The situation is similar to air being distinct from all that it pervades. It is akin to panentheism and it surely is not pantheism.
BTW. I feel that this discussion may not be fit in this thread.