首楞嚴義疏注經 8.195

8.195

初句躡前也；十方下正顯一切眾生皆我心性，心性無外，攝無不周。我之本性既圓滿成就，眾生皆爾，故云不失。此得同體大悲，是故然也。

The opening phrase steps from the preceding. From "ten directions" downward directly reveals that all sentient beings are one's own mind-nature; the mind-nature has no exterior, embracing everything completely. When one's own original nature is consummately accomplished, so too are all sentient beings; hence it says "without losing [sentient beings]." This is attaining great compassion of identical substance, hence it is so.