“虚”指世界或者心灵的一种状态。大体有两种不同的含义：其一，指世界的本原，万物皆由虚无中来。但古人对“虚”的这一含义又有不同理解：或认为“虚”就是空虚无有;或认为“虚”指“气”的存在状态，因为“气”的存在隐微无形，故以“虚”称之，但并非完全空无。其二，指虚静的或没有成见的内心状态。

　　Xu refers to a state of the cosmos or a state of mind. Basically, it has two different meanings. The first refers to the origin of the universe, indicating that everything originates from xu, Different ancient thinkers have different interpretations of this notion: Some take xu as being devoid of anything; others believe it is the state of existence of qi (气). Because qi is invisible and formless, It is said to be empty, but not a vacuum totally devoid of anything. The second meaning of xu refers to a state of mind that is peaceful, not preoccupied or simply free of any preconceptions.

　　Examples

　　太虚无形，气之本体。(张载《正蒙•太和》)

　　(“虚”是无形的，是“气”的本来状态。)

　　Xu is formless; it is the original state of qi. (Zhang Zai: Enlightement Through Confucian Teachings)

　　唯道集虚，虚者心斋也。(《庄子•人间世》)

　　(只有道汇集并呈现于虚静的心灵，“虚”就是心斋。)

　　Dao gathers and presents itself in an unoccupied and peaceful mind; being unoccupied means the pure state of the mind. (Zhuangzi)