Particle physicist Heinrich Päs presents a bold idea: fundamentally, everything in the universe is an aspect of one unified whole. This idea, called monism, has a rich 3000-year history: Plato believed that all is one, but monism was later rejected as irrational and suppressed as a heresy by the medieval Church. Nevertheless, monism persisted, inspiring Enlightenment science and Romantic poetry. Päs shows how monism could inspire physics today, how it could slice through the intellectual stagnation that has bogged down progress in modern physics and help science achieve the grand theory of everything that it has been chasing for decades. Blending physics, philosophy, and the history of ideas, The One is an epic, mind-expanding journey through millennia of human thought and into the nature of reality itself.