 The Han dynasty collapsed in 220 CE, leading to a long period of fragmentation, instability, and uncertainty. The dominant philosophical movement is known as Xuanxue 玄學, “Profound Learning.” The term xuan means dark, obscure, or profound, but it also has a sense of what precedes any division, as it is used in the first chapter of the Laozi. The best known works from this movement are the commentaries by Wang Bi (226–249) (on the Laozi and Yi Jing) and by Guo Xiang (?–312 CE) (on the  Zhuangzi).[9]  Because of the centrality of the Laozi and Zhuangzi, the movement is sometimes known in English as “Neo-Daoism.” Different philosophers held different positions, but the core metaphysical issue was how to understand dao as ultimate ground, particularly how to interpret descriptions of dao as no-being (wu 無) and how to understand the relations between dao and the concrete world of experience.