Born in Higo Province, Kumamoto Domain (present-day Kumamoto City), he studied at the domain school Jishukan and then developed his own distinctive philosophy of practical learning. He fused the loyalist thought of the Mito school with Western science, technology, and political philosophy, developing concrete policy arguments for national enrichment, military strength, and industrial development. Recruited by Matsudaira Shungaku (Echizen domain) as a political adviser in 1858, he supported Shungaku's enlightened domain reforms. Katsu Kaishu, Sakamoto Ryoma, and many other loyalist figures were influenced by his political thought. He was a forward-thinking thinker who held the grand ideal of 'applying the way of the Three Sage Kings (Yao, Shun, and Yu) to the whole world' and evaluated Western concepts of republicanism and democracy from within a Confucian framework. After the Meiji Restoration he served as an adviser to the new government, but on January 5, 1869, he was assassinated in Kyoto by expulsion-faction assailants. He was sixty. His advanced political thought had a major influence on the Meiji government's modernization policies. In particular, his emphasis on 'public deliberation' (kouki) provided the philosophical underpinning for the later Freedom and People's Rights Movement and the development of the parliamentary system.

An intellectual who developed practical policy arguments while remaining idealistic. His unique theory of modernization—not simply imitating Western institutions but fusing them with Eastern moral values—possesses a far-sightedness that transcends his era. His frank words and actions sometimes created friction with those around him.

His contribution as political adviser to Matsudaira Shungaku influenced the Bakumatsu political situation. His political thought emphasizing 'public deliberation' provided the philosophical foundation for Meiji constitutional politics and the Freedom and People's Rights Movement. His advanced political philosophy fusing Eastern and Western thought is still highly regarded by researchers today.