Visuddhimagga XIV-63

442. Rūpaparicchedalakkhaṇā ākāsadhātu, rūpapariyantappakāsanarasā, rūpamariyādāpaccupaṭṭhānā, asamphuṭṭhabhāvacchiddavivarabhāvapaccupaṭṭhānā vā, paricchinnarūpapadaṭṭhānā.

（十六）「虚空界」──有与色划定界限的特相。有显示色的边际的功用（味）。以色的界限为现状（现起）；或以（四大种）不接触的状态及孔隙的状态为现状。以区划了的色为近因（足处）。

Ñ(XIV,63): 16. The space element has the characteristic of delimiting matter. Its function is to display the boundaries of matter. It is manifested as the confines of matter, or it is manifested as untouchedness, as the state of gaps and apertures (cf. Dhs. §638). Its proximate cause is the matter delimited.

Yāya paricchinnesu rūpesu idamito uddhamadho tiriyanti ca hoti.

因为由这（虚空界）区划了色，我们才起了「这是上这是下这是横度」的概念。

Ñ: And it is on account of it that one can say of material things delimited that 'this is above, below, around, that'.