In 1901-02, American psychologist William James was invited to give the Gifford Lectures at the University of Edinburgh. They were published under the title The Varieties of Religious Experience. Two of those lectures, 16 and 17, were dedicated to the topic of mysticism.
Citing testimony after testimony, beginning with witnesses contemporary to his time and then branching out to include others, James offered his audience what he believed were the common features of mystical experience. One of the first is what he calls a noetic quality, a kind of intellectual “objectivity” which his exemplars experienced as a characteristic of the phenomenon. “Although so similar to states of feeling,” says James, “mystical states seem to those who experience them to be also states of knowledge. They are states of insight into depths of truth unplumbed by the discursive intellect.” This is reminiscent of Harris’ evocations of geometric theorems.
Another feature that I single out as particularly germane to the transcendent materialism that I believe truly supports and explains these experiences is its cosmic nature: an identification with the totality of things which entails a corresponding reduction in the sense of self. James quotes a contemporary Canadian psychiatrist:
“The prime characteristic … is a consciousness of the cosmos, that is, of the life and order of the universe. Along with the consciousness of the cosmos there occurs an intellectual enlightenment which alone would place the individual on a new plane of existence — would make him almost a member of a new species.
… Among other things, I did not merely come to believe, but I saw that the universe is not composed of dead matter, but is, on the contrary, a living Presence; I became conscious in myself of eternal life. It was not a conviction that I would have eternal life, but a consciousness that I possessed eternal life then; I saw that all men are immortal; that the cosmic order is such that without any peradventure all things work together for the good of each and all; that the foundation principle of the world, of all the worlds, is what we call love, and that the happiness of each and all is in the long run absolutely certain.
In that time the consciousness of God’s nearness came to me sometimes. I say God, to describe what is indescribable. A presence, I might say, yet that is too suggestive of personality, and the moments of which I speak did not hold the consciousness of a personality, but something in myself made me feel myself a part of something bigger than I, that was controlling. I felt myself one with the grass, the trees, birds, insects, everything in Nature. I exulted in the mere fact of existence, of being a part of it all — the drizzling rain, the shadows of the clouds, the tree-trunks, and so on. In the years following, such moments continued to come, but I wanted them constantly. I knew so well the satisfaction of losing self in a perception of supreme power and love, that I was unhappy because that perception was not constant.
From German Lutheran Jacob Boehme (+1624) he adds this testimony translated by Edward Taylor in 1691:
For I saw and knew the being of all things … the descent and original of the world and of all creatures through the divine wisdom. I knew and saw in myself all the three worlds, the external and visible world being of a procreation or extern birth from both the internal and spiritual worlds; and I saw and knew the whole working essence, in the evil and in the good, and the mutual original and existence, and likewise how the fruitful bearing womb of eternity brought forth. … I could very hardly apprehend the same in my external man and set it down with the pen. For I had a thorough view of the universe as in a chaos, wherein all things are couched and wrapt up, but it was impossible for me to explicate the same.
And to Boehme I will add Johann Angelus Silesius (+1677) who was raised Lutheran and was a follower of Boehme:
In God All is God
In God all is God; the simplest little worm is as much in God as thousands of Gods.
I Am as Vast as God
I am as vast as God; there is nothing in the world, O miracle! — that can shut me up in myself.
As Much as God
I am as much as God; there isn’t a grain of dust I do not share — believe me — with Him entirely.
You Must be Sun
I must be sun, and paint with my own rays the color-free Sea of total Godhead
Similarly, St Teresa of Avila:
One day, being in prayer it was granted me to perceive in one instant how all things are seen and contained in God. I did not perceive them in their proper form, and nevertheless the view I had of them was of a sovereign clearness, and has remained vividly impressed upon my soul. It is one of the most signal of all the graces which the Lord has granted me…. The view was so subtle and delicate that the understanding cannot grasp it. 
These are descriptions of experiences. In most cases the subjects quoted assumed that the ground of this cosmic sense was the “Pure Spirit” that has been traditionally identified as the Western “God.” That assumption was intensified in the 19th century when Idealism of the Hegelian variety was the predominant philosophical preference. But that began to change with James. Self-labeled a “radical empiricist,” James believed that reality was “neutral,” i.e., one substance that was as much mind as matter, i.e., both were made of the same “stuff.”
To say that the ground of our sense of the sacred corresponds to the traditional notion of “God” is hardly confirmed by these descriptions. Even more bizarre are the projections offered by another of James’ citations, Pseudo-Dionysius, a sixth century Syriac Christian monk — projections which derived from the monk’s contemplative experience. Please note: the author is talking about what was commonly assumed to be “God:”
The cause of all things is neither soul nor intellect; nor has it imagination, opinion, or reason, or intelligence; nor is it spoken or thought. It is neither number, nor order, nor magnitude, nor littleness, nor equality, nor inequality, nor similarity, nor dissimilarity. It neither stands, nor moves, nor rests…. It is neither essence, nor eternity, nor time. Even intellectual contact does not belong to it. It is neither science nor truth. It is not even royalty or wisdom; not one; not unity; not divinity or goodness; nor even spirit as we know it …
To these extraordinary assertions I will add what I think are some of the most remarkable of all: statements about “God” made by mediaeval Christian mystic Johannes Eckhart (+1327) that I have frequently cited in my books. Please be aware that in the following passage from his sermon “Blessed are the Poor in Spirit,” Eckhart uses the word “God” in two different senses. Most often it should be enclosed in quotation marks as I have just done, because it refers to a relationship. The in se reality, on the other hand, is never really named:
When I flowed forth from God, all creatures declared: “There is a God”; but this cannot make me blessed [i.e., divinized. Christians since ancient times claimed the goal of “sanctifying grace” was divinization], for with this I acknowledge myself as a creature. But in my breaking through, where I stand free of my own will, of God’s will, of all His works and of God Himself, then I am above all creatures and I am neither God nor creature, but I am that which I was and shall remain forevermore … this breaking through guarantees to me that I and God are one. Then I am what I was, then I neither wax nor wane, for then I am the unmoved cause that moves all things.
… If one wants to be truly poor, he must be as free from his creature-will as when he had not yet been born. For by the everlasting truth, as long as you will to do God’s will and yearn for eternity and God, you are not really poor; for he is poor who wills nothing, knows nothing and wants nothing.
Back in the Womb from which I came, I had no God and merely was myself. I did not will or desire anything, for I was pure being, a knower of myself by divine truth. Then I wanted myself and nothing else. And what I wanted I was, and what I was I wanted, and thus I existed untrammeled by God or anything else. But when I parted from my free will and received my created being, then I had a God. For before there were creatures, God was not God, but rather, he was what he was. When creatures came to be and took on creaturely being, then God was no longer God as he is in himself, but God as he is with creatures.
Now, we say that God, in so far as he is only God, is not the highest goal of creation, nor is his fullness of being as great as that of the least of creatures, themselves in God. … Therefore we pray that we may be rid of God, and taking the truth, break through into eternity, where the highest angels and souls too, are like what I was in my primal existence, when I wanted what I was and I was what I wanted. Accordingly, a person ought to be poor, willing as little and wanting as little as when he did not exist. . . .
The authorities say that God is a being, an intelligent being who knows everything. But I say that God is neither a being, nor intelligent and he does not “know” either this or that. God is free of everything and therefore he is everything. He then who is to be poor in spirit … knows nothing of God, or creatures, or himself. …
Thus far I have said that he is poor who does not want to fulfill the will of God but who so lives that he is empty of his own will and the will of God, as much so as when he did not exist. Next we said that he is poor who knows nothing of the action of God in himself. … But the third poverty is the most inward and real … it consists in that a man has nothing.
… If it is the case that a man is emptied of things, creatures, himself and God, and if still God could find a place in him to act, then we say: as long as that exists, this man is not poor with the most intimate poverty … since true poverty of spirit requires that man shall be emptied of ‘God’ and all his works, so that if God wants to act in the soul, he himself must be the place in which he acts … he would himself be the scene of action, for God is the one who acts within himself. It is here in this poverty, that man regains the eternal being that once he was, now is, and evermore shall be.
… Therefore I pray God that he may quit me of God, for unconditioned being is above God and all distinctions. It was here that I was myself, wanted myself, and knew myself to be this person, and therefore I am my own first cause, both of my eternal being and of my temporal being. To this end I was born, and by virtue of my birth being eternal, I shall never die. It is of the nature of this eternal birth that I have been eternally, that I am now, and shall be forever. For what I am as a temporal creature is to die and come to nothingness, for it came with time and with time it will pass away. In my eternal birth, however, everything was begotten, I was my own first cause as well as the first cause of everything else. If I had willed it neither I nor the world would have come to be. If I had not been, there would have been no God. …” 
I dare say Christians have never heard this kind of talk before. Such descriptions of “God” are not consistent with the traditional notions of a transcendent “theist” God, but they are compatible with a pan-entheism based on transcendent materialism.
Eckhart is meditating on “life before birth” looking for clues about “life after death.” It’s not difficult to understand how he got there. If you assume that the “soul” is a metaphysical “substance” of some type that is not made of matter, not made of parts, therefore cannot die and decompose, then it is not dependent on the body; it cannot have “come to be” with the life of the body otherwise it would “cease to be” at its death. The “soul,” therefore, must have existed before birth.
Eckhart’s meditation about the pre-existent “soul” only makes sense in a Platonic matter-spirit universe. But if you assume, as I do, the worldview of a transcendent materialism, all the elements of the experience described by Eckhart remain in place without the metaphysical inconsistencies: the material organism is completely one with the source of existence — matter’s energy — and therefore the organism’s rational intelligence can identify itself with it; the individual is subaltern to the totality. “Participation-in-being” is not some convoluted mental gymnastics that magically extracts “reality” from human ideas and imagination, but simply articulates direct observation: we are part and parcel of the homogeneous mass of material energy. The breakthrough is spontaneous because the middle term — an “Idea-God” (esse) remote from everything else that exists who even “donates” an alien esse ad extram and cannot be reached without mediation — has been eliminated. “God” is now identical with matter’s existential energy which all things share; there is no remote inaccessible “God” that can only be reached by using the sacramental bridge provided by the Church. “God” as the symbolic personification of the existential life force, is immediately accessible to all, everywhere and at all times because we are THAT — we ARE what “God” is, and Eckhart’s effusive statements about being the “first cause of everything” and Silesius’ claims to be as vast and as much as God suddenly make perfect sense.
I want to emphasize: Eckhart makes no mention of mediation by the Church. We have to recognize that the “God” who is immanent and accessible as esse, which is the Thomist / Aristotelian element of his vision, is completely contrary to a Platonic / Augustinian remote spirit-“God,” inaccessible to matter by nature and alienated from humankind by sin. How can Eckhart leave out “divinization” through the Catholic sacraments? I claim it’s because his breakthrough put him beyond that Platonic worldview and the “other side” of the breakthrough is a pan-entheist universe grounded in esse —for Eckhart an idea, but for me matter’s energy to be-here.
What I am saying is that the universal experience of complete oneness with the Source of all things is directly possible only in a transcendent materialist worldview (or in a monist idealism), but not in the alienated Platonic dualist metaphysical vision underlying traditional Christianity requiring the Church’s bridgework and Eckhart’s breakthrough. The fact that Eckhart’s experience, described without reference to Church or sacraments, conflates so remarkably with the experiences of non-Christians around the world, speaks to their having a common basis with which the Western theories of a remote inaccessible “God” do not concur.
The point of these reflections is to take mystical experience seriously and allow it to generate reasonable hypotheses about the nature of reality and the source of our sense of the Sacred. It calls for a “doctrine of ‘God’” that is so different from the traditional anthropomorphic caricature that the word “God” can no longer be used. Correlatively, if mystical experience proves contradictory to accepted assumptions, the doubts introduced cannot be dismissed or ignored. They must be either resolved satisfactorily or a search for alternatives launched. The universal testimony of the mystics, minimally, calls into question two things: (1) the traditional Christian notions about the unattainable “otherness” of the creator of the cosmos, and (2) reductionist mechanistic materialism ― the two horns of the dualist dilemma. It invites a sincere and unbiased exploration of other explanations.
James, William, Varieties of Religious Experience, a Study in Human Nature (p. 280). Kindle Edition.
 Ibid., p. 294
 ibid., pp. 290-291
 ibid., Boehme and Teresa, p. 302
. Pseudo-Dionysius, T. Davidson’s translation, in Journal of Speculative Philosophy, 1893, vol. xxii., p. 399.
. Johannes Eckhart, sermon: “Blessed are the Poor in Spirit,” quoted in Walshe, M., Meister Eckhart, German Sermons and Treatises, London, Watkins, 1979 vol 2: p.275 (emphasis mine).