Is Ākāśa Within the Three Realms or Beyond Them?

The reasoning for this issue is as follows: First, clarify the concepts and characteristics of empty space and the Three Realms along with the six sense faculties. Then, by determining whether empty space belongs to the dharmas of the Three Realms, one can arrive at a correct and true conclusion. The meaning of "empty space" (虛空) is emptiness or space, primarily constituted by the seed of the space element. The empty space formed by the seed of the space element is also called space, characterized by its capacity to contain and support, possessing actual function—able to contain and support material form-dharmas. Since it is constituted by the seed of the space element, one of the Four Great Elements, empty space is a form-dharma; it has characteristics, namely the characteristic of emptiness. Empty space is a form-dharma, belonging to the category of "form at the boundary of form" (色邊色); it is perceived by eye-consciousness. Form-dharmas perceived by eye-consciousness necessarily have characteristics, as explained in the Yogācārabhūmi-śāstra.

However, the characteristics of "form at the boundary of form" differ from those of other form-dharmas. Upon opening the eyes, one immediately knows whether the space before them is empty or not. This empty form-dharma is perceived by eye-consciousness, just as non-empty form-dharmas are also perceived by eye-consciousness. Empty space has its own characteristic of emptiness, while non-empty material form-dharmas have their own non-empty characteristics. If empty space lacked characteristics, how could eye-consciousness recognize whether it is empty or not?

The definition of the Three Realms is that they consist of dharmas that exist, are not empty, and include form-dharmas, mind-dharmas, as well as dharmas that are neither form nor mind. Mind-dharmas and form-dharmas are two distinct categories, mutually exclusive. However, form-dharmas are form-dharmas within the mind; form-dharmas are not external to the mind. Empty space is also a dharma within the mind, not external to it. Empty space is also an existent dharma; it is not empty. Therefore, empty space belongs to the dharmas of the Three Realms. There is only one dharma not within the Three Realms: Suchness or Self-nature (真如自性). Since empty space is a form-dharma, has function (it can support material form-dharmas), empty space is certainly within the Three Realms, not outside them. There is definitely no empty space outside the Three Realms. The mind-dharmas of the seven consciousnesses, although not within empty space, are still 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 there are no dharmas outside the Three Realms. An Arhat who has attained nirvana has no form-dharmas or the mind-dharmas of the seven consciousnesses; thus, they do not abide within empty space. Only form-dharmas can abide within empty space, and these are still within the Three Realms.

Empty space is the "emptiness" of space. "Tortoise hair and rabbit horns" represent the "nothingness" of complete non-existence and non-being; this is not the "emptiness" of empty space. Therefore, the issue of whether "tortoise hair and rabbit horns" are inside or outside the Three Realms does not arise. Empty space has no boundary; the worlds of the ten directions are all within empty space. The Three Realms have boundaries; existence and non-existence (nothingness) mark the boundaries of the Three Realms. The Three Realms are suffering. Reaching the "end of suffering" (苦邊), where there are no longer form-dharmas or the mind-dharmas of the seven consciousnesses, is the state of Nirvana without residue (無餘涅槃)—this is the boundary of the Three Realms. Nirvana and non-Nirvana constitute the boundary line.