Volume 10 of the Śūraṅgama Sūtra: The Inverted Subtle and Profound Conceptions of the Vijñāna Skandha

This explains that the skandha of formations is a subtle and hidden delusion not easily perceived. "Subtle" refers to what is obscure and secret, while "hidden" refers to what is concealed—both describing the phenomena of birth, death, and transformation within the physical body that are difficult for people to detect. These phenomena cannot be controlled by subjective conscious awareness; they arise due to karmic forces, manifesting as various changes in the body over the flow of time. If karma ceases, one would attain immortality and eternal youth, like beings in the Pure Land or celestial beings in the realms of desire and form.

The subtle and hidden delusion is still delusion; the imperceptible skandha of formations is also a product of delusion. Thoughts arise and cease without cease, like subtle waves flowing imperceptibly, with birth, aging, sickness, and death never interrupted. If delusions are extinguished, the skandha of formations ceases to arise, leading to the stillness of Nirvana—eternity, bliss, true self, and purity.