But he also dabbled in the more spiritual, wrote extensively about notions of Tai Chi, which is really a Taoist or even a pre-Taoist idea, thinking about what is the fundamental nature of the universe, and that Tai Chi itself is the great ultimate, sometimes represented by the yin-yang symbol. Really, this is showing Tai Chi divided into this dualistic nature between yin and yang, how at the center of each or at the extreme of each, they become the other. He writes about qi, this life force or energy. And once again, these are ideas he takes from Taoism. But in Neo-Confucian thought, there's this idea of you shouldn't just detach yourself from physical reality, you should study it. There's an order, there's a logic to the universe that could be understood. And because of the influence of the Neo-Confucians, especially their focus on belief in order and logic of the universe, it's no coincidence that the Sung Dynasty saw some of the major technological advancements, not just of China, but of the world, advancements that really put China at the technological forefront.