The Śūraṅgama Sūtra, Fascicle Ten: The Inverted, Subtle, and Exquisite Thinking of the Aggregate of Consciousness

This says that the aggregate of volitional formations is a subtle and hidden delusion, difficult to perceive. "Hidden" (幽) means dark and secret, and "concealed" (隐) means hidden away; both refer to the phenomena of arising, ceasing, and change of the physical body that are not easily perceived by people. These phenomena are not something the subjective conscious mind can control; they are due to the power of karma—the various changes that the body undergoes as time flows on. If karmic power were to disappear, one would enjoy eternal youth and never age, for example, the sentient beings in the land of bliss (sukhāvatī) and the devas of the desire realm and form realm.

The hidden and subtle delusion is also a delusion; the aggregate of volitional formations, which is not easy to perceive, is also a product of delusion, flowing and shifting thought after thought without pause—the preceding thought arises and the following thought ceases, like subtle waves flowing secretly, with birth, aging, sickness, and death never ceasing even for a moment. If delusion is extinguished, the aggregate of volitional formations will not arise, and there will be the tranquility of nirvāṇa, with permanence, bliss, self, and purity.