Besides this writing which is the only one left by Dharma in our possession at present, we have the _Laṅkāvatāra-sūtra_, _Vajrasamādhi-sūtra_, and _Vajracchedikā-sūtra_, through which we can also have a glimpse into the central teaching of Bodhi-Dharma. Zen, unlike other schools of Buddhism, has no particular Sutras to be called the “foundation canon” on which its followers would base the principal tenets of their school; but Dharma recommended the _Laṅkāvatāra_ to his first disciple Hui-k‘ê (Yeka), as containing the teaching most intimately related to Zen, and after him this scriptural writing came to be studied chiefly by Zen scholars.[f88] As to the importance of the _Vajrasamādhi_ as expounding the philosophy of Zen, we can easily understand it from Dharma’s own reference to the Sutra in his writing as was already pointed out. With regard to the _Vajracchedikā-sūtra_, most people think of it as having nothing to do with Zen prior to the fifth patriarch, Hung-jên (Gunin); for it was he who for the first time introduced it among his own disciples, while Dharma himself made no allusion whatever to this, one of the most popular Buddhist texts in China. But according to Hui-nêng’s Preface to the _Vajracchedikā_, which is still preserved, “ever since the coming-west of Dharma he wanted to propagate the meaning of this Sutra and lead people to understand the Reason and to see into the Nature.” If this were actually the case, Dharma, to say the least, must have had some knowledge of this Sutra from the very beginning of his career in China, and the connection in a way between this and Zen must have been more fundamental than that between the _Laṅkāvatāra_ and Zen. The prevalent notion then that the _Vajracchedikā_ came only in vogue after Hung-jên and Hui-nêng must be revised. Whatever this may be, the _Laṅkāvatāra_ is too difficult a material for popular consumption, and it was natural that this Sutra came to be gradually superseded by the _Vajracchedikā_ as Zen gained more and more in power and influence. As one of the Sutras belonging to the Prajñāpāramitā class of Buddhist literature, the teaching of the _Vajracchedikā_ was comparatively simple and had something much akin to the Laotzŭan ideas of emptiness and non-doing. It was not hard for the average Chinese to follow its philosophy of Śūnyatā, in fact this agreed well with a certain aspect of Chinese thought.[f89]