All five consciousnesses function in the realm of desire, that is, on the first ground. On the second ground eye-, ear-, and body-consciousness function, but nose-consciousness and tongue-consciousness do not function, because at that level (i.e., at the level of the first dhyana), the smell and taste objects of perception do not exist, nor does the type of morsel-nourishment which is connected with smell and taste. In the first dhyana nourishment takes place through contact rather than through the eating of meals comprised of morsels of food (the first of the four types).
Ordinarily we think only of nourishing our bodies through the intake of ordinary food and drink; however, the Buddhadharma distinguishes Four Kinds of Nourishment:
1) Mouthfuls. This kind is distinguished by the nose and tongue. Its substance is perceived through smell, taste, and contact. This ordinary food, bodily nutriment, changes and decays. It can be gross, solid, or fine. This kind of nourishment takes place only in the realm of desire.
2) Mental Contact. This kind nourishes the body by contact with joyous situations. In other words that the first six consciousnesses--seeing, hearing, smelling, tasting, touching, and cognizing--can have special value as food. Nourishment by contact does not exist independent of the fourth kind of nourishment (see below).
3) Volition. When associated with the sixth consciousness, volition can function as food. It is characterized by desire for perceptual objects, thus aiding the five perceptual organs in attaining their objects. It occurs in all three realms, but does not exist independent of the fourth kind of nourishment. Therefore, the sixth consciousness in itself can have special value as food.
4) Consciousness. According to the Mahayana it refers to the eighth consciousness. It indicates that consciousness is capable of nourishing the bodily life of sentient beings. Life feeds off the eighth consciousness, the basic life force or life energy. When that life-energy is exhausted, death occurs.
One of the basic ideas here is that the nourishment needed by a being corresponds to its level of vital and conscious life. Coarse food is effective nourishment for a coarse organism but is of no use for a fine one. Higher and higher levels of life and consciousness must be fed with progressively finer and finer kinds of nourishment. Yet in the conditioned world even life on the finest and highest level of consciousness must “eat”.
Beyond the first dhyana, that is, on the third through ninth grounds, none of the five consciousnesses arise.
The Grounds on Which the Consciousnesses Arise