This essay asserts that the Laṅkāvatāra-sūtra plays an indispensible role — which cannot be fulfilled by any other sūtra or treatise—in analyzing the philosophical background of the Èrrù sìxíng lùn (“Two Entrances and Four Practice”). This assertion is explored through the medium of two important aspects of the Èrrù sìxíng lùn, namely the “mechanisms of practice” and the “process of falling into delusion.” The Laṅkāvatāra-sūtra expounds the view that it is one's own heart that gives shape to reality and that that shape, in turn, is taken to be an independently-existing reality (Skt. svacittadṛśyamātra, Chi. Zìxīnxiànliáng). And this forms the basis of delusion. This process of falling into delusion, itself, actually becomes one with the mechanism of practice, in which shape is done away with and there is a return to the original, shapeless-heart, state of nature. This philosophical construction forms the basis for the entirety of the Èrrù sìxíng lùn's content. One can see that—regardless of the authenticity of the Laṅkāvatāra-sūtra legend (i.e., that the sūtra passed down through the generations of Zen practitioners)—there is no question in regard to the importance of the Laṅkāvatāra-sūtra to its believers: as seen through the lens of the picture painted in the Èrrù sìxíng lùn. And the Zìxīnxiànliáng logic of the Laṅkāvatāra-sūtra is, in fact, an integral part of Huìkě's teaching.