In terms of spiritual awakening, I understand the "I" witness behind the mind, body and ego but can it be just the rational part of the brain recognizing the mind, body and ego?

Rationality is a good thing in the world of the spirit. Reason is not our enemy, it is our friend. If the reasoning functions of our brains feel comfortable dividing who we are into mind, body, and ego as it observes us, that is fine.

But I would like my rationality to take me even further, beyond mind, body, and their attendant egos. There comes a point when even reason will take us to a place that goes beyond rationality. Because rationality can take us only so far.

At this point, reason has no footing in the world of the Spirit. That is because reasoning is linear: This caused that. That caused this. This happened because of that. All this is in time because one thing follows another in a sequence. We need to find reasons and we look into the past to see the causes. We look into the future to see the effects.
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In the world of spiritual awakening, time does not exist. Nor does space. Causality, as the rational mind knows it, does not exist. It is all happening at once. It is beyond the rational, it is beyond the laws of Newtonian physics, and it cannot be explained adequately in any language that exists on this planet today. So it’s OK for the rational part of the brain to recognize all of our components, but it is a limited perspective.