The essence of Buber's I-Thou experience, it seems to me, is the recognition of oneness, a sense that the perceived and the perceiver are part of a mutual necessity. Of course, none of us is uniquely necessary for the completion of the whole. If the sperm that become half of me had lost the race, the universe would not have been substantially changed. One star more or less makes no difference to a galaxy. I depend utterly on the Sun; the Sun does not depend on me, but there is a sense in which we are in this together. It is the deeply-felt perception of wholeness that is the defining experience of the religious naturalist.
In the I-Thou experience we are lifted out of the grayness of It into what Virginia Woolf called "moments of being." I hesitate to use the term "mystical experience"; that sounds rather too pretentious. There is nothing supernatural or out-of-body about it. It can be as simple as "the certain minor light" that sometimes leaps out of inanimate objects, as in Sylvia Plath's Black Rook in Rainy Weather, or Gerard Manley Hopkins' "shining from shook foil" -- a synaptic firestorm in the brain triggered by the senses, available to all. We are changed by the experience, elevated -- sometimes, for particularly fragile souls, dangerously so.
We don't give the source of the experience a name. Any name is idolatrous, especially the name of God, burdened as it is with anthropomorphism. To experience the Thou we must be skeptical but open, knowledgeable but ignorant, walking wary, ready to be struck dumb. "Creation happens to us," writes Buber, "burns itself into us, recasts us in the burning." We tremble. We submit.
Knowledge enhances the experience of the Thou, especially the reliable knowledge of science, but knowledge alone will not evoke the experience. We walk the shore where knowledge is lapped by mystery, one foot on dry land, the other in the sea. "Men do not find God if they stay in the world," says Buber. "They do not find Him if they leave the world."
"'Here world, there God' is the language of It," he writes; "'God in the world" is another language of It; but to eliminate or leave behind nothing at all, to include the whole world in the Thou, to give the world its due and its truth, to include nothing beside God but everything in him -- this is full and complete relation."
It is clear that nothing is lost in Buber's notion of the eternal Thou if we reject the words that have become heavy with anthropomorphic meaning. Indeed, Buber's "God" has no namable qualities. In Buber's I and Thou some of us found a path away from the dualisms that so bedevil traditional religions: natural/supernatural, matter/spirit, body/soul. Dualism, as Buber suggests, is the language of It. His Thou, like the Thou of religious naturalism, is the undivided all that we experience partly illuminated by knowledge and partly through a glass darkly.