首楞嚴義疏注經 8.581

8.581

未離欲心，逢境暫遘，漸薄於下，故云去無思憶。逢欲猶交，故云動少；去無思憶，故云靜多。斯行又增身則又勝，故生時分。焰摩云時分，此是空居初天也。

They have not yet left the mind of desire; encountering circumstances, they mate temporarily, but it is gradually thinner than below, hence it is said 'upon leaving have no recollection.' Encountering desire, they still engage, hence 'seldom active'; upon leaving, they have no recollection, hence 'mostly still.' As this practice increases further, their forms are further superior; hence they are born in the Time-Division. Yāma means 'Time-Division'; this is the first of the heavens dwelling in empty space.