"Lower functions have a dual aspect of positive and negative. The higher emotional and higher intellectual centers do not have that duality."
The “non-duality” of higher centers is a “relative non-duality” and is still part of Duality. Duality being the “ten thousand things”. The manifestation. The universe. Subject-object. Perceiver-perceived. Experience.
We are mixing different concepts from different traditions while using the same terms. These traditions are describing different models and they are not interchangeable.
The term “non-duality” mean something different in Non-Dual teachings compared to the 4th Way or the Michael teachings.
Duality exists in all planes and all centers. “Non-Dual Truth”, as expounded by Buddha, Lao Tzu, Jesus, Nisargadatta, Ramana and many others, has no relation to planes of existence or with centers higher or lower. It is exactly the realization of the transcendence of those “centers” and the multitude of experience they provide, that is equal to Non-Dual Truth.
No higher center can grasp Non-Dual Truth. That concept or pursuit is a trap that many seekers fall into, or a detour they take, because they read in some book that higher centers is the “holy grail”. Add to that some personal experiences of higher centers and we are “hooked”. This is not my idea. This is documented in various traditions. Even Gurdjieff seems to have acknowledged this in his last book. Life is real only then when “I Am”. Not: “Life is real only then when I have this or that state of consciousness”. Find the Real “I Am”.
The planes of existence are duality. The evolution and ascension of the soul are duality. Any experience, perception, higher or lower is duality.
The Realization of Non-Duality has no specific relation to higher-centers and is available at all times. If that was not the case we would all be spiritually doomed on the physical plane.