Is 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 space (ākāśa) and the three realms' six faculties; then, by determining whether space belongs to the dharmas of the three realms, one can arrive at a truthful and correct conclusion. Space means emptiness, void—mainly constituted by the seed of the space element. The space constituted by the space element seed is also called space; its characteristic is the capacity to contain and support, having actual function, able to contain and support material form (rūpa-dharma). Since it is constituted by the space element seed among the four great elements, space is material form, having a mark—the mark of emptiness. Space is material form; it belongs to the boundary‑aspect of form, and is seen by the eye consciousness. Material form seen by the eye consciousness necessarily has a mark; this is discussed in the Yogācārabhūmi-śāstra.

It is only that the mark of the boundary‑aspect of form differs somewhat from the mark of material form. Open your eyes and look, and you immediately know whether what is before you is empty or not empty. This material form of emptiness is seen by the eye consciousness, and the material form that is not empty is also seen by the eye consciousness. 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 there are dharmas—not empty—comprising material form, mind, and dharmas that are neither material form nor mind. Mind and material form are two boundaries that do not contain each other, yet material form is material form within the mind; material form is not a dharma outside the mind. Space is likewise a dharma within the mind, not outside the mind. Space is also an existing dharma, not empty; therefore 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 space is material form and has function, able to support material form, space is of course within the three realms, not outside the three realms; outside the three realms there is certainly no space. Although the mind-dharmas of the seven consciousnesses are not within space, they are dharmas within the three realms; the eighth consciousness, suchness, 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 mind-dharmas of the seven consciousnesses, and thus does not abide in space; anything that could abide in space would be material form, and would also not be within the three realms.

Space is the emptiness of space, whereas a turtle's hair and a hare's horns are the nothingness of having nothing at all and the emptiness of nonexistence—not the emptiness of space. Therefore, turtle's hair and hare's horns do not involve the question of being within or outside the three realms. Space has no boundaries; the worlds of the ten directions are all within 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 and no more mind-dharmas of the seven consciousnesses, is nirvāṇa without remainder (anupadhiśeṣa-nirvāṇa)—that is the boundary of the three realms. Whether one is in nirvāṇa or not in nirvāṇa is precisely the boundary line.