The year is 1174.

Gravity, oxygen, and electricity have not been discovered.

Clocks, calculus, and the printing press have not been invented.

Earth is the center of the universe, encircled by heavenly bodies whose motions are ministered by angels.

Most people never live past their thirties.

Medicine abides by the Greek theory of the humors and treats all ailments with a combination of bloodletting, herbal tinctures, amputation, and the King’s Touch.

No university will educate a woman. In fact, no university exists.

At seventy-six, Hildegard of Bingen — poet, painter, healer, composer, philosopher, mystic, medical writer — has just finished writing and illustrating her third and farthest-seeing book: The Book of Divine Works, chronicling seven years of prophetic visions. God had first begun speaking to her in “the voice of the Living Light” when she was three, but she never suffered the hubris of a self-appointed prophet — rather, she considered herself “a totally uneducated human being,” a “wretched and fragile creature,” who is merely a channel for divine wisdom. She may be the Western world’s first great crusader against dualism — in the sermons she delivered to priests, bishops, abbots, and ordinary people all over present-day Germany and Switzerland, she preached that “God is Reason,” that “Reason is the root” from which “the resounding Word blooms,” but also that “from the heart comes healing,” that we apprehend the world and its wisdom most clearly through the intuitions of the “inner eye” and “inner ear.”

Hildegard was fifty-six when she began receiving the vision that would become her Book of Divine Works. On its pages, between writings about birds and trees and stones and stars, between reckonings with the nature of eternity and the fundaments of love, she conceptualizes something the word for which would not be coined for another seven centuries: ecology.

Long before Alexander von Humboldt invented modern nature with his recognition that “in this great chain of causes and effects, no single fact can be considered in isolation,” before John Muir insisted that “when we try to pick out anything by itself, we find it hitched to everything else in the universe,” Hildegard places at the center of her cosmology the notion of viriditas, from the Latin for “green” — a greening life-force pervading the world, mirrored in the virtues that enlush the soul.

Human beings, she writes, are “co-creators with God” in the operations of nature. We must cooperate with one another in the task of protecting and nourishing this interconnected creation, and we must do so by integrating the rational and the intuitive in us. Hildegard’s human being is “the fragile vessel where soul and reason are active,” filled with “the fullness of time.”

In one of her visions, collected in the wonderful translation Hildegard of Bingen’s Book of Divine Works: With Letters and Songs (public library), she paints a menacing picture of a world in which we have grown disconnected from the greening life-force of our own souls. Seven centuries before Eunice Newton Foote discovered greenhouse gasses, and an epoch before we had any sense of climate change or our own hand in it, Hildegard prophecies:

Then the greening power of the virtues faded away, and all justice entered upon a period of decline. As a result, the greening power of life on Earth was reduced in every seed because the upper region of the air was altered in a way contrary to its first destiny. Summer now became subject to a contradictory chill while winter often experienced a paradoxical warmth. There occurred on Earth times of drought and dampness… As a result, many people asserted that the Last Day was near at hand.

She was unambiguous about what stands between us and such fate:

If… we give up the green vitality of [our] virtues and surrender to the drought of our indolence, so that we do not have the sap of life and the greening power of good deeds, then the power of our very soul will begin to fade and dry up.

And yet Hildegard believed in “the green vitality of human volition,” believed that “the soul knows what is good and what is harmful.” By integrating our rational faculty with our heart-honed intuition, by refusing to dishonor our own souls, we have within us the power to revivify this Earth. It what may be the clearest, most succinct manifesto for climate action, she writes:

Our thinking affects our greening power… The soul is the green life-force of the flesh… When we humans work in accord with the strivings of our soul, all our deeds turn out well.

This, indeed, is the beating heart of Hildegard’s viriditas: the insistence that the stewardship of Earth’s life-force is not merely our moral obligation to the universe but our spiritual duty to our own souls. And this can only be so — the words holy and whole share a Latin root; if an ecological conscience is a way of seeing the world whole, it is a way of seeing its holiness, of seeing our own holiness — not above it, but nested within it. Rachel Carson knew this when, picking up Hildegard’s torch eight centuries later to catalyze the modern environmental movement, she observed that “there is in us a deeply seated response to the natural universe, which is part of our humanity,” that the task now before humanity is “to prove its maturity and its mastery — not of nature, but of itself.” It was Hildegard who gave us the original model of poetic ecology.