Yamaga Sokō was a Japanese Confucianist of the kogakuha, or "school of ancient learning," and codifier of the ethics of the military class, Bushidō, the "way of the warrior." He was born in Aizu, Fukushima prefecture. At nine he entered the school of Hayashi Razan in Edo (Tokyo), where he learned the official Zhu Xi doctrine. Interested in military science, he became a master of it. He taught it first at the castle of Lord Asano of Akō (Hyogo prefecture) and later in Edo, where the novelty of his advocating the use of firearms attracted many followers. In 1666 he wrote Seikyō yōroku (The essence of Confucianism), a blunt critique of Zhu Xi's ideas. For this and for his innovations in military science, he incurred the wrath of his two former teachers, Hayashi and the military expert Hōjō Ujinaga...