Selected Lectures on the Sūtra of the Collection of the Father and Son (Twenty-Four)

Original text: 大王。若色变坏。一切皆空。所以者何。是虚空界。本无尽故。安静不动。犹如涅盘。遍一切处。无有障碍。

Explanation: Great King, when material form (rūpa) deteriorates and perishes, it becomes empty nothingness—that is the realm of empty space (ākāśa-dhātu). Why is it that when material form disappears, everything becomes empty? Because the realm of empty space has no end and no boundaries, everything can become empty. Moreover, empty space is inherently tranquil and unmoving, neither arising nor ceasing, neither increasing nor decreasing, just like the nature of nirvāṇa. Empty space pervades all places, and there is no dharma that can obstruct it.

When things perish, they become empty space. For example, when a house is destroyed, that place entirely becomes empty space; when the earth is destroyed, the place of the earth entirely becomes empty space; when the trichiliocosm (three-thousandfold great world system) is destroyed, the entire trichiliocosm becomes empty space, with nothing at all within that empty space.

Empty space is tranquil and unmoving. Why is it unmoving? Because it is a provisional designation for the absence of things. Material objects are also unreal, yet objects can be moved, whereas no one can move empty space even a little. If a very tall building is constructed in empty space, can the empty space be shifted? Empty space does not move; no one can move empty space, because empty space has no material substance and no boundaries—how could it be moved? Within empty space there is no material form (rūpa); it is nothing at all, empty nothingness. We constantly need empty space and emptiness in order to move our bodies and place objects, because empty space has no obstruction, and sentient beings need this quality of non-obstruction for moving about and storing things.

Some people say that air is also empty and should, like empty space, be boundless and immovable. But in fact there is a great difference between air and empty space. Air appears on the surface to be without substance—invisible and intangible—yet instruments can detect the presence or absence of air. Air does contain matter: it holds the various gases we need, all of which are forms (rūpa) composed of the four great elements, and therefore air can flow and move. What sentient beings need from air, above all, is the oxygen within it.