Why Does Excessive Thinking Lead to Fatigue?

Question: The physical body is composed of the four great elements. The four great elements within the physical body are also in constant flux. Humans absorb the four great elements from outside the body through eating, breathing, etc., transforming them into nutrients composed of the four great elements required by the body. Simultaneously, the four great elements within the human body are constantly depleting, being expelled through various excretions. Only the tathāgatagarbha possesses the function to operate all these processes. When a person thinks, the tathāgatagarbha continuously takes up the four great elements at the subtle sense faculties to manifest dharma objects, supplying them for the consciousness and the manas to discern. Therefore, excessive thinking consumes too much of the four great element nutrients at the subtle sense faculties, causing the brain to feel fatigued. If the supply of the four great element nutrients to the subtle sense faculties is insufficient, thinking capacity declines. When a person is overly fatigued, they need to sleep, allowing the tathāgatagarbha to operate the four great elements to replenish the four great element nutrients of the subtle sense faculties. Is this correct?

Answer: Dharmic objects of the "exclusive mental realm" (dúyǐng jìng) do not consume the four great element substances of the subtle sense faculties. However, dharmic objects included in the mental sense field (dharmāyatana) do consume the four great element nutrients within the subtle sense faculties. Nevertheless, when the manas and consciousness operate, they necessarily stimulate the subtle sense faculties and neurons, releasing bioelectricity, which consumes the four great element nutrients. Consequently, excessive thinking causes the brain to feel fatigued, requiring rest to allow energy within the brain to accumulate again.

When the mind-consciousness stirs, the brain nerves activate, releasing bioelectricity and expending energy. Even when entering samādhi where the six consciousnesses are still, the manas continues to discern subtly, operating in a hidden manner, still consuming a small amount of the four great element nutrients. The more subtle the activity of the manas, the less it consumes; the deeper the samādhi, the more subtle the activity of the manas, and the less it consumes. Upon entering the "non-perception samādhi" (wúxiǎng dìng), one can remain in samādhi for several kalpas or longer without needing food or drink, but the body will still become emaciated.