This book explores one of philosophy's most fundamental questions: what is a thing, and how does it change? Challenging the traditional divide between things and processes, Zhen Liang brings together continental philosophy, process metaphysics, quantum mechanics, and contemporary theoretical physics to develop an original "ontology of systems" that accounts for both the stability and fluidity of reality. Through engagements with Newton, Leibniz, Whitehead, modern physics, and Daoist thought, the book traces the historical origins of substance-based thinking before proposing an alternative framework in which things, systems, and processes are understood as inseparable. Accessible to both scholars and intellectually curious readers, this interdisciplinary study opens a distinctive dialogue between philosophy and science while offering a fresh perspective on the nature of reality, change, and the direction of human thought.