The Lankavatara Sutra
The Lankavatara was not written by a Zen master, nor did it come out of the Zen tradition, but it was the primary scripture of the first two hundred-years of Chan. The major concept it advances is that of Mind, characterized by D. T. Suzuki as:
. . . absolute mind, to be distinguished from an empirical mind which is the subject of psychological study. When it begins with a capital letter, it is the ultimate reality on which the entire world of individual objects depends for its value.
On the question of Mind, the Lankavatara has the following to say:
. . . the ignorant and the simple minded, not knowing that the world is what is seen of Mind itself, cling to the multitudinous of external objects, cling to the notions of being and non-being, oneness and otherness, bothness and not-bothness, existence and non-existence, eternity and non-eternity .
According to the Lankavatara, the world and our perception of it are both part of a larger conceptual entity. The teachings of the Lankavatara cast the gravest doubt on the actual existence of the things we think we see. Discrimination between oneself and the rest of the world can only be false, since both are merely manifestations of the same encompassing essence, Mind. Our perception is too easily deceived, and this is the reason we must not implicitly trust the images that reach our consciousness.
. . . [I]t is like those water bubbles in a rainfall which have the appearance of crystal gems, and the ignorant taking them for real crystal gems run after them .... They are no more than water bubbles, they are not gems, nor are they not-gems, because of their being so comprehended [by one party] and not being so comprehended [by another].
Reality lies beyond these petty discriminations. The intellect, too, is powerless to distinguish the real from the illusory, since all things are both and neither at the same time. This conviction of the Lankavatara remained at the core of Zen, even after the sutra itself was supplanted by simpler, more easily approached literary works.