摩訶止觀 6.16

6.16

若無想天，留色滅心，故名「無想」。情謂無想，具足想在。例如斷事障，性障猶存，終不出色，此名外道天。前破見心，見心久去，當不生此天。或為因緣事，心起此定，即有三假等。亦用四觀破之，相續、相待亦如是。

As for the Heaven of Non-Conception, it retains form and annihilates mind, hence it is called "non-conception". Feeling imagines it to be non-conception, yet conception is fully present. By analogy, this is like severing the obstacles of phenomenal activity while the obstacles of intrinsic nature still remain, so that one ultimately does not transcend form; this is called an externalist heaven. Previously, the mind of view-delusions was broken; since the mind of view-delusions has long been removed, one would not be born into this heaven. Or, due to phenomenal causes and conditions, if the mind gives rise to this concentration, the three provisionalities and so forth are immediately present. One also uses the four contemplations to break it; the provisionalities of continuity and relativity are likewise treated in this way.