首楞嚴義疏注經 2.236

2.236

汝疑見性縮斷，要在一義決定。見性之義猶如虛空，虛空豈有方圓而可在耶！此明真見周遍，無有方所，如彼虛空。故《涅槃》云「有常之法遍一切處，虛空常故無處不遍，如來亦爾遍一切處，是故為常。無常之法此有彼無，如來不爾，是故為常。」

You doubt whether the nature of seeing shrinks or is severed, demanding determination in a single meaning. The meaning of the nature of seeing is like space: how could space have squareness or roundness such that it could have a location? This clarifies that the true seeing is all-pervading and has no location, like that space. Thus the Nirvāṇa [Sūtra] says: 'Dharmas that are permanent pervade all places; because space is permanent, there is no place it does not pervade. The Tathāgata is also like this, pervading all places; therefore he is permanent. Impermanent dharmas exist here and are absent there; the Tathāgata is not like this, and therefore is permanent.'