本文參考初唐道教典籍《玄珠錄》，探討道教「空」的概念。《玄珠錄》將「空」歸類為道之特徵，討論「道」其實體（substance）與「空」之關係。首先，就現象的外顯而言，與道脫離，會產生各種表象，而與道結合，則會揭露天地萬有的本性（true nature）。依《玄珠錄》所言，這種本性與「空」的性質是一致的。雖然道其本質是空，卻可以感應萬物。道唯有空，才能感應並產生各式各樣、五花八門的萬物，否則就只能受限於某種性質（nature），而不足以涵蓋既有現象的全面性質。其次，本書運用「四句」——有、無、亦有亦無、非有非無——主張道既處於這些存在情境之中，又在這些存在情境之外。這其實是「得道」的知識論基礎。但在此同時，本書認為道必須克服絕對（being absolute）之侷限。換句話說，道無疑是一種實體，但這個實體的概念不完全符合任何把實體視為單一性質的傳統意義。除了「四句」，本文也提出「是一」和「非一」的概念來釐清何謂道。其論證如下：當道「是一」，便不具有相對性。當道「非一」，表示道超越了普遍性、特殊性，相對性和絕對性。第三，《玄珠錄》表示，只要根本上將感知除去，便可臻於「空」，從而「得道」。簡而言之，即使因為在本體論上沒有區隔，在知識論上也無法區分，而把道描繪成「空」，當吾人達到「不朽」時，仍然必須藉由「道」來確立永恆的境界。

This article deals with the Taoist notion of emptiness with reference to an early Tang period Taoist text entitled Xuanzh Lu. As the text assigns emptiness to the attribute of Tao, the relation between Tao as a substance and emptiness is discussed. First of all, as far as the manifestation of phenomena is concerned, separation from Tao will give rise to all appearances, whereas integration with Tao will unveil the true nature of all beings. This true nature, according to the text, conforms to the virtue of emptiness. Although Tao as a substance is empty, it is capable of responding to things. Only by being “empty” is the Tao able to “respond” and produce the multifarious myriad things, otherwise it will have to confine itself to a certain nature and hence will not be sufficient to embrace the whole nature of existing phenomena in itself. Secondly, by using the “four verses” such as Being, Nonbeing, both Being and Nonbeing, both non-Being and non-Nonbeing, the text holds that Tao is situated in these states of existence as well as beyond these states of existence. This is actually the epistemological foundation of “attaining the Tao.” But at the same time, the text deems that Tao has to surmount the limitation of being absolute. In other words, no doubt Tao is a substance, but this notion of substance does not really comply with any conventional meaning which considers substance as a single-natured entity. Apart from the “four verses,” the text also draws out the notions of “One” and “non-One” to clarify Tao. The argument goes like this: When the Tao is “One,” it is thus free from relativity. When the Tao is“non-One,” it means that the Tao is something beyond universality and particularity, relativity and absoluteness. Thirdly, the Xuanzhu Lu suggests that with the instant elimination of perceptions, emptiness can be achieved, thus Tao is attained. In short, even though Tao is portrayed as emptiness because it is ontologically non-differentiated and epistemologically non-differentiatable, it is needed in order to secure the state of eternity when one reaches immortality.