Green Mass is a meditation on-and with-twelfth-century Christian mystic and polymath Saint Hildegard of Bingen. Attending to Hildegard's vegetal vision, which greens theological tradition and imbues plant life with spirit, philosopher Michael Marder uncovers a verdant mode of thinking. The book stages a fresh encounter between present-day and premodern concerns, ecology and theology, philosophy and mysticism, the material and the spiritual, in word and sound. Hildegard's lush notion of viriditas, the vegetal power of creation, is emblematic of her deeply entwined understanding of physical reality and spiritual elevation. From blossoming flora to burning desert, Marder plays with the symphonic multiplicity of meanings in her thought, listening to the resonances between the ardency of holy fire and the aridity of a world aflame. Across Hildegard's cosmos, we hear the anarchic proliferation of her ecological theology, in which both God and greening are circular, without beginning or end. Introduced with a foreword by philosopher Marcia Sa Cavalcante Schuback and accompanied by cellist Peter Schuback's musical movements, which echo both Hildegard's own compositions and key themes in each chapter of the book, this multifaceted work creates a resonance chamber, in which to discover the living world anew. The original compositions accompanying each chapter are available free for streaming and for download at www.sup.org/greenmass