What sentient beings rely upon is unreliable

The supreme faculty (rūpa-prasāda) is located at the back of the brain; it is where the mental dharmas and material form are connected. When the four great elements of the supreme faculty fall into disharmony—such as cerebellar atrophy and degenerative brain disease—the resulting impairments of movement, cognition, and memory all arise from the aging and pathological degeneration of the supreme faculty. Then the six consciousnesses, which are mental dharmas, cannot arise and function normally, and sentient beings can easily die. Both cerebral hypoxia and cerebellar atrophy can destroy the supreme faculty and obstruct the arising of the six consciousnesses and all kinds of cognitive activities. Human life is extremely fragile: a single breath failing to come means cerebral hypoxia and brain death, bringing life to an end. Why is it easy to attain meditative concentration (samādhi) through practicing deep breathing? Deep breathing ensures the supply of oxygen to the brain and benefits physical health. With the brain unobstructed and the six consciousnesses comfortable, there is no need to struggle against or alter the physical body, and thus it becomes easier to attain concentration.

From this we can see that the functions of the five aggregates (pañcaskandha) are of dependent arising (paratantra) in nature; they cannot be self-governing, cannot stand on their own, and are extremely illusory and unreal. The normal activities of the five aggregates depend on so many external conditions—lacking even one of them will not do. Such a body of the five aggregates is utterly unreliable and far too insubstantial; therefore none of it is the self. What sentient beings depend upon is all unreliable. If one could manage to depend on nothing at all, then one would be fearless. It would be wonderful if in the future we could cultivate to the point of not relying on any external environmental conditions, so that arising-and-ceasing things would no longer affect us. At that point one would be almost ageless and deathless—presumably this requires the eighth bhūmi or above. We should pursue and rely upon an unchanging, constant dharma that is reliable and does not require external conditions.