This book was partially written based on a translation, with additions and modifications, of the Japanese book “Fish also reason the self” by M. Kohda, published by Chikuma-Shobo Inc. in 2021. New additions include Chaps. 8 and 9 , which reexamine intelligence and self-awareness in a wide range of animals based on our recent findings. As you read through this book, you will have noticed the detailed and surprising content on animal insight, including higher levels of self-awareness and reason in fish, that have not appeared in any scientific documents, overturning once assumed common knowledge. We hope that by reading our book, readers will reconsider the common sense that has come from anthropocentrism.