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; then, by determining whether empty space belongs to the dharmas of the three realms, one can arrive at an accurate and correct conclusion. Empty space means emptiness, space, and it is mainly constituted by the seed of the space element (ākāśa-dhātu). 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 support material form (rūpa). Since it is constituted by the seed of the space element among the four great elements, empty space is material form, has a mark—it is the mark of emptiness. Empty space is material form; it belongs to the boundary-aspect of form (the boundary-aspect of form), and is what the eye consciousness sees. Material form seen by the eye consciousness necessarily has a mark, as explained in the Yogācārabhūmi-śāstra. 

It is only that the mark of the boundary-aspect of form differs somewhat from the mark of material form. With one opening of the eyes, one knows whether what is before one is empty or not empty. This material form of emptiness is what the eye consciousness sees, and the material form that is not empty is also what the eye consciousness sees. Emptiness has the mark of emptiness, and the material form that is not empty has the mark of not being empty. If emptiness had no mark, how could the eye consciousness distinguish whether it is empty or not empty? 

The definition of the three realms is that they have dharmas, are not empty, and include material form, mental dharmas, and dharmas that are neither material nor mental. Mental dharmas and material form are two distinct boundaries that do not contain each other, but material form is material form within the mind; material form is not a dharma outside the mind, and empty space is likewise a dharma within the mind, not outside the mind. Empty space is also a dharma that exists, not empty; therefore empty space belongs to the dharmas of the three realms. There is only one dharma not in the three realms: the self-nature of suchness (tathatā). 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. Although the mental dharmas of the seven consciousnesses are not within empty space, they are dharmas within the three realms; the eighth consciousness, suchness (tathatā), 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 who has entered nirvāṇa no longer has material form or the mental dharmas of the seven consciousnesses, and thus does not abide in empty space; whatever can abide in empty space is material form, and is also not within the three realms.

Empty space is the emptiness of space, whereas a turtle's hair and a rabbit's horns are the nothingness of having nothing at all and the emptiness of nonexistence—not the emptiness of empty space. Therefore turtle's hair and rabbit'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 nothingness constitute the boundaries of the three realms. The three realms are suffering; reaching the end of suffering, with no more material form or the mental dharmas of the seven consciousnesses, is nirvāṇa without remainder (Anupadhiśeṣa-nirvāṇa)—that is the boundary of the three realms. Whether in nirvāṇa or not in nirvāṇa—that is the boundary line.