Is Empty Space Within the Three Realms or Outside the Three Realms?

The approach to this question is as follows: first, clarify the concepts and characteristics of empty space (ākāśa) and the three realms' six faculties, and then determine whether empty space belongs to the dharmas of the three realms; in this way, an accurate and correct conclusion can be reached. Empty space means emptiness, spatial void, and it is mainly constituted by the seed of the space element. The empty space constituted by the seed of the space element is also called space; its characteristic is that it can contain and support—it has actual function, being able to contain and bear material form (rūpa-dharma). Since it is constituted by the seed of the space element among the four great elements, empty space is material form, having a mark—it is simply the mark of being empty and devoid of things.

Empty space is material form; it belongs to the form that is the boundary-aspect of form (rūpa-paryanta-rūpa), and it is seen by the eye consciousness. Material form seen by the eye consciousness must have a mark; it is only that empty space has the mark of being empty and having nothing, whereas material form has the mark of being non-empty and having form. This part is discussed in the Yogācārabhūmi-śāstra. The mark of the boundary-aspect of form differs somewhat from the mark of material form: with one open-eyed look, one knows whether what is before one is empty or non-empty. The material form that is empty is seen by the eye consciousness, and the material form that is non-empty is also seen by the eye consciousness; the empty has the mark of emptiness, and the non-empty material form has the mark of non-emptiness. If empty space had no mark, how could the eye consciousness discern whether it is empty or non-empty?

The definition of the three realms is that they are existent dharmas, not empty; they comprise material form (rūpa-dharma), mental dharmas (citta-dharma), and dharmas that are neither form nor mind. Mental dharmas and material form are two distinct boundaries that do not contain each other, but material form is the material form within the mind, not a dharma outside the mind; empty space is likewise a dharma within the mind, not outside the mind. Empty space is also an existent dharma, not a non-existent dharma; therefore empty space belongs to the dharmas of the three realms. There is only one dharma that is not within the three realms: the self-natured true mind. Since empty space is material form and has function, being able to support material form, empty space is of course within the three realms, not outside the three realms; outside the three realms there is certainly no empty space.

Empty space is the emptiness of space, whereas a turtle's hair and a hare's horns are the nothingness of absolutely nothing and the emptiness of non-existence—not the emptiness of empty space; therefore turtle's hair and hare's horns do not involve the question of being within or outside the three realms. Empty space has no boundaries; the worlds of the ten directions are all within empty space. The three realms have boundaries: existence and absolute nothingness constitute the boundaries of the three realms. The three realms are suffering; reaching the end of suffering, when there is no more material form and no more mental dharmas of the seven consciousnesses, that is nirvāṇa without remainder (Anupadhiśeṣa-nirvāṇa)—that is the boundary of the three realms; nirvāṇa and non-nirvāṇa are the boundary line. Although the mental dharmas of the seven consciousnesses are not within empty space, they are dharmas within the three realms; the tathāgatagarbha is neither a dharma within the three realms nor a dharma outside the three realms, because outside the three realms there are no dharmas. An arhat in nirvāṇa, having no material form and no mental dharmas of the seven consciousnesses, does not abide within empty space; whatever can abide within empty space is material form, and such an arhat is also not within the three realms.