The eighteen realms are the treasury of the Tathagata

Sutra:

“Moreover, Ananda, why do I say that the eighteen realms are basically the wonderful nature of true suchness, the treasury of the Tathagata?

Commentary:

Shakyamuni Buddha said to Ananda, “How is it that the eighteen realms are basically the wonderful nature of true suchness, the treasury of the Tathagata?”

What are the eighteen realms? They are the eyes, ears, nose, tongue, body, and mind - that makes six - together with forms, sounds, smells, tastes, objects of touch, and dharmas, the six defiling objects, which are six more realms. To them are added the six consciousnesses: the eye consciousness, the ear consciousness, the nose consciousness, the tongue consciousness, the body consciousness, and the mind consciousness. These three groups of six are the eighteen realms. The six sense organs are matched to the six defiling objects, and between them are produced the six consciousnesses. The consciousnesses are defined as that which makes distinctions. The sense organs are defined as that which grows, in that they are grown on our bodies. The defiling objects are defined by their quality of defilement. They are unclean, and they defile the nature of the six organs. When the six organs are matched with the six defiling objects, consciousnesses arise. The eyes see forms and make distinctions among them as being attractive or unattractive. They like the forms or they do not, and thus give rise to discriminations. With the ears it is the same: they hear sounds as pleasing or displeasing. Either they like a sound or they do not like it. The production of such distinctions is called the ear consciousness. The nose smells fragrance and stench. The two defiling objects of fragrance and stench are distinguished as such. You may like some odors and dislike others, and in this way you give rise to a nose consciousness. The tongue distinguishes flavors. Because the organ of the tongue is matched to the defiling objects of flavors, there is the discrimination of flavor. Flavors are either pleasant or disgusting - you either like them or you don’t. The body organ is matched with defiling objects of touch - smooth or abrasive, coarse or fine, various kinds of sensations, either pleasant or unpleasant. The body organ matched with the defiling objects of touch produces a consciousness which discriminates these sensations.

The organ of the mind is matched with the defiling objects of dharmas. The five defiling objects just discussed - forms, sounds, smells, tastes, and objects of touch - all have form and appearance. Only the defiling objects of dharmas are without form or appearance. There is no representation of them. Nonetheless, when the organ of the mind is matched with the defiling objects of dharmas, discrimination is produced in the mind, and so the mind also has a consciousness. In this way the six organs matched with the six defiling objects produce the six consciousnesses, and together they make up the eighteen realms. Although they are divided into eighteen realms, they are entirely contained within the wonderful nature of true suchness of the treasury of the Tathagata.