The Relationship Between the Physical Body and the Four Great Elements

Time, this false dharma, is quite marvelous. If a person could fly to the heavenly realm, stay for a few minutes, and then fly back, hundreds of years might have already passed in the human world—several lifetimes of human lifespans would have gone by—yet this person would still be as young as before, not aged at all. Why is this so?
Whether one ages or not depends on the physical body. The physical body is composed of particles of the four great elements; through arising, ceasing, and change, the body continuously matures, develops, and transforms. The four great elements constitute the qi and blood needed to sustain the physical body. If the qi and blood are vital and flowing, and the body receives ample nourishment, it will not easily mature and age, and the person will appear young. Qigong and various cultivation practices can all make the qi and blood active and circulating, nourishing the physical body; therefore, those who practice well generally look young. Time reveals the arising, ceasing, and change of the four great elements—the longer the time, the more evident the arising, ceasing, and change, and the more mature and aged the body becomes. In short, youth and aging lie in the development and change of the organization of the four great elements. 

Why do the material bodies of devas not age? Because the composition of the four great elements in a deva's material body differs from that in a human's material body. The four great elements of humans are coarse and heavy (dausthulya), while those of devas are subtle and refined. Devas can fly, but humans cannot. After humans ascend to the heavens, in accordance with the living environment of the heavenly realm, they absorb the particles of the four great elements of that realm, and the four great elements of their material body will also change, so the body becomes close to that of the devas. When devas come to the human world and absorb the particles of the four great elements of the human world, their material bodies draw closer to those of humans; the four great elements also change and become close to those of humans. For example, when a deva of the Light-Sound Heaven (Ābhāsvara) of the second dhyāna comes to the human world and tastes the flavors of the earth, the four great elements of the material body immediately change, and the deva can no longer fly freely and cannot return to the heavens. 

When the qi is full, one has no thought of food. Qi is material energy composed of the four great elements. After one sits in meditation and cultivates meditative concentration (samādhi) until the qi is sufficient, one no longer feels hungry and can go without eating or eat less; the body feels light and comfortable. Because the four great elements of qi differ from those of food and drink—the four great elements of qi are fine and light, and sufficient qi can make the physical body float upward, while the four great elements of food and drink are coarse and heavy and make the body heavy. Therefore, eating less and cultivating meditative concentration more will make the body healthier. No matter how nutritious food and drink may be, they are still coarse material form (rūpa), not the best thing, and not as nourishing as qi.