A particle physicist makes the scientific case for monism, the ancient idea about the universe that says, all is One In The One, particle physicist Heinrich Päs presents a bold idea: fundamentally, everything in the universe is an aspect of one unified whole. The idea, called monism, has a rich three-thousand-year history: Plato believed that "all is one" before monism was rejected as irrational and suppressed as a heresy by the medieval Church. Nevertheless, monism persisted, inspiring Enlightenment science and Romantic poetry. Päs aims to show how monism could inspire physics today, how it could slice through the intellectual stagnation that has bogged down progress in modern physics and help the field achieve the grand theory of everything 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.