Thanks for this very thoughtful response. You've definitely read more about this than I have. I still think my perspective gets through the language and metaphor to the root of the issue, though.
The quote above is where I think we fundamentally disagree. I do think mystical experiences are a type of experience, and that all experiences consist of a pattern of chemical activity in the brain. (It is reductionist, but that doesn't make such experiences any less meaningful to us.)
This area we are in agreement on. If you are experiencing it, chemicals and the brain are involved. Mystical experiences are a type of experience. Nothing wrong with using a reductionist lens to analyse. Just don't stop there and call that the truth of what it is. That's nothing thinking enough.
To then attribute these experiences to anything real outside of the brain needs its own evidence.
I did pick up on this in your last response, but didn't want to dig into this at that point. I will now, as it marks a considerable contrast in thinking about these things.
I hear this view that externalizes the world, that externalizes God. It's a very dualistic perspective, dividing the world and the self into subject/object divisions. I think this comes from the Christian West in its traditional-theism view of the Divine. God is viewed as external to creation, outside of it. So hearing someone speak of experiences which transcend ordinary mundane reality for them, it is viewed dualistically, as an "other" reality to this one. That is foundationally what makes it dualistic, and this reality seem supernatural when it exposes things one normally cannot see.
All this is a fiction of the mind, rooted in language and dualistic thought. What happens to that division, that divide between "God", or the experience of the Absolute, or Reality in a mystical experience, is metaphorically speaking the curtains drawn open and discarded, and what is "out there" or perceived as external to us, is experienced to be intrinsically linked to us, and you to everything outside yourself. In the highest states, there is one organic whole, and you are it, and it is you.
Even science bears this out theoretically, this interconnectedness of all reality. If you deal with the complexity sciences, systems theory, chaos theory, quantum mechanics, et al, more and more it appears scientifically exactly that everything is connected in ways beyond our imaginings in the past, in a Newtonian view of reality as its sole scientific lens of perception. Rationally, we can see it. Mystically we can experience it. We can see it with the perceptual eye, as we look at that same tree, and see unity.
Language divides reality into objects and separates us from the world. When someone punches through that veil of obstruction, say in a mystical state of awareness, what is described sounds "supernatural", only because it is outside one's own lived perception of reality, as reality itself. What that must look like, is strange indeed! So the mind imagines the supernatural, and mythologies are born.
What is contained within the magical and mythical symbols of religious expressions, are not pointing to another world, even though its language describes it thusly. That's simply the filter of the dualistic mind, taking that which is "transcendent" to one's normal reality such as that is, and imagining it as other worldly, magical, etc. And in reality, it is! That is in fact what the beauty of science is revealing. It's beyond imagining. Something the mystic, who tap into the fabric of that Reality, have expressed all along. But all of that, is this Reality you and I live in every moment of existence, even now.
Some call that Fabric, "God", and then paint a human, dualistic face upon it, and see it as other to themselves.
Edited to add:
I realize looking at this this morning, this rather stream of consciousness response above missed my original point it began with. To clarify, while some may attribute the source of the experience to something "external" to oneself, all experiences originate within us. If someone has a transcendent experience in response to some perceived external stimuli, the experience does not come from outside of us. It come up from within us in response to something.
So if someone experiences "God", that experience comes from within, in response to something perceived as external to us, like seeing a sunset. In reality, there is no division, and God is in all. The experience at its peak realizes this, as touched on above. What may be perceived as external to us, is actually a part of us, and ultimately is us. We are in essence, responding to a recognition of our own true nature, and that experience defines what transcendence means. We transcend ourselves. Not sure if that helped, or muddied this further.
I'm not saying things outside the brain weren't proximate causes of the experience, like looking at a painting, or smelling incense, or entering a contemplative state, but I don't see any evidence that the mystical experience itself correlates to some special attribute of reality.
It's a state of perceptual awareness. It's seeing reality through a different set of eyes. It's seeing what is there all along, but only obscured by perceptual lenses which filter it, more or less. What it correlates to is the deeper and higher potentials of human conscious awareness. At the highest states, the mind perceives what the duller senses cannot bring into focus.
I like this quote from Einstein, whose mind was so intellectual it passed into the mystical, as his many writings contain. Right here in this quote, he is touching upon that "emotion" as he calls it, which is to use that word, transcendent to our reasoning minds. This is a beautiful quote from him.
“The most beautiful thing we can experience is the mysterious. It is the source of all true art and science. He to whom the emotion is a stranger, who can no longer pause to wonder and stand wrapped in awe, is as good as dead —his eyes are closed. The insight into the mystery of life, coupled though it be with fear, has also given rise to religion. To know what is impenetrable to us really exists, manifesting itself as the highest wisdom and the most radiant beauty, which our dull faculties can comprehend only in their most primitive forms—this knowledge, this feeling is at the center of true religiousness.”
- Albert Einstein, Living Philosophies
That Mystery, is not apart from us.
Part of the problem may be that I'm not sure what you mean by "transcendent." If you mean a state of mind, then I could agree. If you mean an aspect of external reality, then it needs its own evidence. Otherwise, it is indistinguishable from all the other things humans imagine, and I prefer not to believe imaginary things.
It is a condition of being and state of mind in which the external world is seen in transcendent light. How's that for an answer?