Kaiho Seiryo (1755–1817) is probably the first Japanese thinker to proclaim the contractual nature of human relationships. I examine in this paper the view of human beings that led him to this conclusion. Giving up previous definitions of humans, Seiryo focuses on the faculty of practical reason. While this leads him to recognize a hierarchy of humans, some having more humanity than others, it also allows him to develop the most modern understanding of social relationship available in his time. His radical reinterpretation of what it is to be a human being is all the more remarkable because it was done with the concepts and ideas provided by the Chinese Classics. Establishing new connections, giving new life to ideas that were never exploited, Seiryo showed it was possible to make sense of modernity without using foreign concepts.