This book proposes and systematically elaborates the "Sentient Principle," a novel paradigm for life sciences. The Sentient Principle traces back to Robert Whytt's (1751) insight into the essence of life, was deepened through Ivan Sechenov's theory of "Systemic Feelings," and achieves theoretical integration on the foundation of Gerald Edelman's Neural Darwinism. The core thesis of the Sentient Principle is that the essence of life lies in the "Sensation-Behavior Loop." Sensation drives behavior, behavior changes sensation, and new sensation drives new behavior in continuous cycles. This loop comprises three core elements: (1) Value System Sensation, providing evaluative standards; (2) Anticipatory Sensation, gradually clarifying in the "Remembered Present"; (3) Sensation Discrepancy, the mismatch between immediate sensation and anticipatory sensation, driving learning and adaptation. The book reveals the complete generative logic from innate to acquired, from biological to social and cognitive layers. Innate "Sensation-Behavior Closed Loops" gradually differentiate into refined acquired "Sensation-Behavior Loops" through Neural Darwinian selection mechanisms, guided by value system sensations. Social and cognitive "sentience" emerges from biological interactions through the unified mechanism of "Resonance-Evaluation-Consolidation." The Sentient Principle systematically reconstructs core concepts in traditional psychology and physiology: needs are dominant value system sensations, emotions are combinations of sensation discrepancy and anticipatory sensation clarity, memory is the consolidation of sensation-behavior traces, and consciousness is the continuous comparison between high-clarity anticipatory sensation and immediate sensation. All these concepts are projections of the "Sensation-Behavior Loop" from different perspectives. This book not only provides a unified theoretical framework but also points toward a practical path: becoming a "Sentient Person"—a lucid self-observer, an active loop regulator, and a warm resonator of life. The Sentient Principle marks a fundamental shift in mind sciences from physics-imitating "cognitive science" toward life-embracing "life science."