釋禪波羅蜜次第法門 8.43

8.43

隨意者，若是遍行，雖在定內，得見諸緣，出禪定時，則觀不相應。今隨意者，隨出入定，觀一切法，任運自成，不由作意，是名隨意。此則略釋觀支相也。

As for 'at-will': in the case of pervasive activity, although within absorption one can see all conditions, upon emerging from dhyāna absorption the observation is not concordant. Now, 'at-will' means that whether emerging from or entering absorption, one observes all phenomena naturally and spontaneously without relying on mental effort; this is called at-will. This is a brief explanation of the characteristics of the vicāra factor.