This book explores the essence of immunity. After an initial review of hypotheses, models, and theories proposed to explain immune phenomena in humans and mice, it summarizes the results from synchronic organism‑level analyses and diachronic analyses tracing phylogeny. These results suggest that immunity is coextensive with life and is equipped with functions similar to the nervous system. Philosophical reflection with reference to Spinoza and Canguilhem suggests immunity is part of the essence of life—and the essence of immunity embraces mental elements with normativity. Approaching the essence of any phenomenon in this way is called "metaphysicalization of science." This book demonstrates the potential of this approach and contributes to a richer understanding of nature.Key Features Reviews the history of immunological theories Discusses and integrates science and philosophy Provides a biological framework for cognition and self vs. nonself Inspired by Auguste Comte’s "The Law of Three Stages"