It is the law of this world that lighter things float to the top and things that are heavy sink to the bottom. For instance, the earth stays below and the water covers it because water is lighter than the earth. Fire stays above water because fire is lighter than water. The wind stays above fire because wind is lighter than fire. The sky exists above wind because it is lighter than wind.

Human beings also consist of these four great elements (earth, water, fire, and wind). When an evil person dies, the wind and fire elements leave the corpse, leaving the water and earth elements behind. The person thus becomes heavy, and the body falls into hell. In the case of a virtuous person, the heavy water and earth elements disappear first, leaving the light elements of fire and wind. The body of such a person thus becomes light, shows the countenance of rebirth in the human or heavenly realm.

Kōnichi Shōnin Gohenji, A Reply to Rev. Kōnichi, Writings of Nichiren Shōnin, Followers II, Volume 7, Page 153-154