in 1939, feng brought out his "xin lixue" ("new rational philosophy", or "neo-lixue"). lixue was a philosophical position of a small group of twelfth-century neo-confucianists (including cheng hao, cheng yi, and zhu xi); feng's book took certain metaphysical notions from their thought and from taoism (such as li and tao), analyzed and developed them in ways that owed much to the western philosophical tradition, and produced a rationalistic neo-confucian metaphysics. he also developed, in the same way, an account of the nature of morality and of the structure of human moral development.