Kuiji’s 窺基 Youzan 幽賛 (T1710) is one of the earliest commentaries on the Heart Sūtra 心経 translated by Xuanzang 玄奘. In the introduction to this text, Kuiji insists, based on the theory of three periods of teaching 三転法輪 in the Saṃdhinirmocana Sūtra 解深密経, that emptiness (śūnya 空) and existence (bhava 有) are not contradictory, but rather neither existent nor non-existent according to the Middle Way 中道.In the commentary he interprets śūnya in terms of the three approaches of: (1) the three non-natures 三無性, (2) negation of the imaginary nature (parikalpita-svabhāva 遍計所執性), and (3) śūnyatā 空性 as the fully accomplished nature (pariniṣpanna-svabhāva 円成実性), based on the theory of three natures 三性 articulated in the Cheng weishi lun 成唯識論.
The Youzan contains Mādhyamika comments on śūnya, which is interpreted from the perspective of the two truths 二諦.However, all of them are criticized from the Yogācāra perspective of the theory of three natures. The remarkable point is this: while Madhyamaka insists that ordinary persons (bāla 凡夫) originally being noble ones (ārya 聖者) depends on the interpretation that form (rūpa 色) is originally emptiness (śūnya 空), Yogācāra insists that there is a difference in ordinary and noble depending on the understanding that there is also difference in form and emptiness.
The difference is clear by the interpretation of practice (caryā 行). Madhyamaka interprets this term to mean “taking nothing as practice,” but the Yogācāras criticize this interpretation, asserting that “there is something as practice.” Kuiji, who was concerned that the Mādhyamika interpretation of śūnya might undermine the theoretical basis for practice, insists that the argument for the usefulness of practice could be supported by the Yogācāra’s expanded interpretation of śūnya.
Thus, it can be said that the emphasis of practice by the Yogācāra interpretation of ‘śūnya’ is a main feature of the Youzan. This explains why the Yogācāra comments on caryā account for the bulk of the arguments made in the Youzan.