首楞嚴義疏注經 8.248

8.248

非染非淨故名為一，離諸虛妄故名為真，常住不變故名為如，約此義邊名為不動。又此地中無分別智任運相續，相用煩惱不能動故，名不動地。此地證得不增減真如，謂此真如離增減執，不隨染淨有增減故，斷無相中作加行障，修願波羅蜜增勝。

Neither defiled nor pure, it is named 'one'; free from all falsity, it is named 'true'; permanently abiding and unalterable, it is named 'as such' (tathatā); in terms of this meaning, it is named 'immovable'. Furthermore, on this ground non-conceptual cognition continues spontaneously, and the afflictions of sign and application cannot move it; hence it is named the Immovable Ground. On this ground, one realizes the tathatā free from increase and decrease, meaning that this tathatā is free from graspings of increase and decrease and does not increase or decrease following defilement and purity; one severs the hindrance of producing effort within the signless and cultivates the vow-pāramitā to supreme excellence.