瑜伽師地論略纂 15.55

15.55

論云「不斷貪等一切煩惱、隨煩惱纏，修四念住，無有是處」者，謂有漏時，謂伏貪故，方起不淨觀等故。又已成就念住，無漏修也。

The treatise says: 'Without severing all afflictions and secondary afflictions such as craving, to cultivate the four smṛtyupasthānas is impossible': this means that during the defiled state, it is because craving is subdued that one then gives rise to aśubha-bhāvanā and so forth. Furthermore, having already accomplished the smṛtyupasthānas, one cultivates them undefiledly.