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, and then determine whether space belongs to the dharmas of the three realms; from this, an accurate and correct conclusion can be drawn. Space means emptiness, void space, and it is mainly constituted by the seed of the space element. The space constituted by the seed of the space element is also called void space; its characteristic is the capacity to contain and support—it has actual function, being able to contain and support material form (rūpa-dharma). Since it is constituted by the seed of the space element among the four great elements, space is material form and has a mark; it is merely the mark of being empty and without anything.

Space is material form, belonging to the form at the boundary of material form, and it is seen by the eye consciousness. Material form seen by the eye consciousness must have a mark; it is only that space has the mark of being empty and having nothing, whereas material form has the mark of being non-empty and having color. This part is discussed in the Yogācārabhūmi-śāstra. The mark of the boundary-aspect of form differs from the mark of ordinary material form. Open your eyes and take a look, and you will know whether what is before you is empty or non-empty. This empty kind of material form is seen by the eye consciousness, and non-empty material form is also seen by the eye consciousness. Emptiness has the mark of emptiness, and non-empty material form has the mark of non-emptiness. If emptiness had no mark, how could the eye consciousness distinguish whether it is empty or non-empty?

The definition of the three realms is that they are existent dharmas, not empty; they contain material form, mental dharmas, and dharmas that are neither material nor mental. Mental dharmas and material form are two distinct boundaries that do not include each other, but material form is the material form within the mind, not a dharma outside the mind. Space is also a dharma within the mind, not outside the mind. Space is also an existent dharma, not a non-existent dharma; therefore, space belongs to the dharmas of the three realms. There is only one dharma not within the three realms: the true mind of self-nature. Since space is material form and has function, being 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.

Space is the emptiness of void space, whereas a turtle's hair and a rabbit's horns are the nothingness of absolute non-existence, an emptiness of non-existence, not the emptiness of space. Therefore, turtle's hair and rabbit's horns do not involve the question of whether they are 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 non-existence constitute the boundaries of the three realms. The three realms are suffering; reaching the end of suffering, when there is no longer material form and the mental dharmas of the seven consciousnesses, is nirvāṇa without remainder (anupadhiśeṣa-nirvāṇa)—this is the boundary of the three realms. Nirvāṇa and non-nirvāṇa constitute the boundary line. Although the mental dharmas of the seven consciousnesses are not within 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 in space; anything that could abide in space would be material form, and such is also not within the three realms.