The Profound Yogācāra Meaning of the Diamond Sutra (39)

When sentient beings' eyes see form, they believe they are truly seeing form; in fact, sentient beings have never seen external form. What the eye faculty contacts is only a false appearance, a shadow. If external form were actually pressed against the eye faculty, one would no longer be able to see that external form. If the eye could truly contact form, then when the eye sees a sharp knife and the blade tip touches the eye, it would certainly pierce and injure the eyeball; when the eye sees fire and the fire touches the eye, it would certainly burn the eyeball; when the eye sees sand and the sand touches the eye, it would certainly get into and irritate the eye. The eye faculty and form cannot make contact. Only when there is a certain distance between form and the eye faculty can the four great elements' particles of the form sense-object be transmitted. It is the tathāgatagarbha that, through the eye faculty, contacts the external form and transforms it into an identical shadow, and only then can it be seen. If form were pressed against the eye faculty with no spatial distance, form could not be seen. The nose faculty, tongue faculty, and body faculty, however, can directly contact the external scent objects, taste objects, and touch objects, without requiring a certain distance or space; the three consciousnesses can then be generated to discern scents, tastes, and tangibles.

The shadow seen by the eye consciousness differs from the material substance of external form; it is not the actual thing. Only then can it be transmitted through the retina and the conducting nerves to the supreme faculty (rūpa-prasāda) in the back of the brain that governs vision. The tathāgatagarbha there brings forth the eye consciousness, and the eye consciousness then discerns the form sense-object. This form appears to be the same as the external form, but in reality it is merely similar. Sentient beings take it to be real, when in fact it is all shadows. By the same token, it is the same with the ear faculty, nose faculty, and tongue faculty: none of them can contact external sounds, scents, or tastes; it is all the tathāgatagarbha that makes contact and then manifests them. Whatever is born is illusory and transformed; this is the principle that all dharmas are created by the mind alone. In short, all marks are false and illusory; if one sees that all marks are no-marks, one sees the Tathāgata. If one can, among the various marks of the five aggregates and eighteen realms, find a tathāgatagarbha that has no mark and is not a mark, that is seeing the Tathāgata—seeing one's true mind, awakening and realizing the true reality.