This book describes the process of making the major breakthrough in the study of animal self-awareness using fish. The discovery led by the author’s team, proving the mirror self-recognition ability of fish, is vividly documented as they share the process of making, testing and verifying hypotheses and developing further hypotheses. The clear experimental results demonstrate the remarkable self-awareness in animals, overturning the conventional view and providing a key to understanding the origin of human self-awareness. Starting from the current understanding of fish brains, individual recognition by its face, the following chapters introduce the series of the authors’ research projects designed to understand mirror self-recognition (MSR) in animals. The sequence of the research into fish’s MSR is documented, including how it started, the failures and successes, and the struggles. Additional tests carried out in response to various criticisms of the work have led to a re-examination of the research methods used prior to the author’s work. The book then addresses the question of exactly when and how some fish recognize themselves in a mirror, exploring the self-awareness and the “mind”, in other word “Eureka moment” in fish. This book points out and overturns the contradictions in conventional wisdom based on anthropocentrism and hypotheses about the evolution of self-awareness, proposing a new hypothesis that the self-awareness of humans and fish will be homologous. The book takes readers on an engaging exploration of the scientific experiments and the remarkable discovery of animal intelligence. Cover illustration : Cleaner fish seeing its own face in a mirror (Photo by Dr. Taiga Kobayashi) This book describes the process of making the major breakthrough in the study of animal self-awareness using fish. The discovery led by the author’s team, proving the mirror self-recognition ability of fish, is vividly documented as they share the process of making, testing and verifying hypotheses and developing further hypotheses. The clear experimental results demonstrate the remarkable self-awareness in animals, overturning the conventional view and providing a key to understanding the origin of human self-awareness. Starting from the current understanding of fish brains, individual recognition by its face, the following chapters introduce the series of the authors’ research projects designed to understand mirror self-recognition (MSR) in animals. The sequence of the research into fish’s MSR is documented, including how it started, the failures and successes, and the struggles. Additional tests carried out in response to various criticisms of the work have led to a re-examination of the research methods used prior to the author’s work. The book then addresses the question of exactly when and how some fish recognize themselves in a mirror, exploring the self-awareness and the “mind”, in other word “Eureka moment” in fish. This book points out and overturns the contradictions in conventional wisdom based on anthropocentrism and hypotheses about the evolution of self-awareness, proposing a new hypothesis that the self-awareness of humans and fish will be homologous. The book takes readers on an engaging exploration of the scientific experiments and the remarkable discovery of animal intelligence. Masanori Kohda is a Professor at Graduate School of Science, Osaka Metropolitan University, Japan. His research interests include evolution and comparative cognitive science of animals, especially the mind and self-awareness. Shumpei Sogawa is a Post-doctoral fellow at Graduate School of Science, Osaka Metropolitan University, Japan. His research interests include animal behavior and comparative cognitive science, especially reciprocal altruism and self-awareness.