In 1799 he became involved in the compilation of the Dai Nihon-shi (Great History of Japan) being undertaken by the Mito school.
In 1825 he wrote his Shinron ("New Theses"), a collection of essays that dealt with issues such as Tokugawa defence policy and how the ships were a threat to Japan. Aizawa also tried to describe conditions in the West and theorize why those states had gained so much control; in his opinion Westerners used religion to enforce conformity of the masses. He also claimed that Christianity was used to secretly subvert native cultures and governments, facilitating military conquest by Europeans. He believed that Japan would need to take up its own state religion and discussed the concept of kokutai ("national polity"), in this context. The Shinron would become an important work for the sonnō jōi movement and his theory of the Kokutai would be developed by future thinkers.
In 1840 Aizawa became the first head of professors of the Mito school's Kōdōkan but was forced to resign in 1844 when Tokugawa Nariaki resigned as domain leader. He later returned to the Kōdōkan.
- Wakabayashi, Bob Tadashi (1986). Anti-Foreignism and Western Learning in Early-Modern Japan: The New Theses of 1825. Harvard University Press. ISBN 0-674-04025-2.
- Josephson, Jason Ā. (2012). The Invention of Religion in Japan. Chicago: University of Chicago Press.
- Josephson, p. 55.