Why Do We Feel Fatigued When Deliberating Excessively?

Question: The physical body is composed of the four great elements, and the four great elements in the body are also constantly flowing and changing. A person absorbs the four great elements from outside the body through eating, breathing, and so on, transforming them into the nutrients composed of the four great elements that the body needs. At the same time, the four great elements in the body are also constantly being depleted and expelled through various excretions. And for all of this functioning, only the tathāgatagarbha has this capability. When a person is thinking, the tathāgatagarbha continuously takes in the four great elements at the supreme faculty and manifests them as mental sense-objects, for the mind consciousness and the manas to cognize. Therefore, when one thinks excessively, the nutrients of the four great elements at the supreme faculty are consumed too much, and the brain will feel fatigued. When the supply of the four great elements' nutrients to the supreme faculty is insufficient, the capacity for thinking declines. When a person is overly fatigued, they need to sleep, letting the tathāgatagarbha operate the four great elements to replenish the nutrients of the four great elements at the supreme faculty. Is it like this?

Answer: The mentally imagined realm of mental sense-objects (subjectively imagined realm) does not require consuming the four great elements of the supreme faculty, whereas the material form included in the mental-object realm does require consuming the nutrients of the four great elements within the supreme faculty. However, when the manas and the mind consciousness are operating, they touch and activate the supreme faculty and the neurons, releasing bioelectricity and consuming the nutrients of the four great elements. Therefore, when one thinks too much, the brain feels fatigued and needs to rest for a while, waiting for the energy in the brain to gather again.

When the conscious mind stirs, the brain's neurons fire, releasing bioelectricity and releasing energy. Even when one enters meditative concentration and the six consciousnesses are not moving, the manas is still engaged in subtle cognition, secretly active, and it also consumes a small amount of the nutrients of the four great elements. The subtler the manas' activity, the less it consumes; the deeper the meditative concentration, the subtler the manas' activity, and the less it consumes. When one enters the concentration of no perception (asaṃjñi-samāpatti), one can remain in concentration for several kalpas or even longer, without needing food or drink, but the body will still become thin and weak.