Commentary on the Prajñāpāramitā Sutra of the Compendium of Father and Son (IX)

What are the eighteen objects of mental observation?

It is said: When the eye has seen a visible form, whether pleasure arises, or distress arises, or one abides in equanimity — similarly, each of the six faculties, in relation to their respective objects, gives rise to these three states: pleasure and so forth. These are called the eighteen objects of mental observation.

Explanation:

"Observation" (伺察) refers to examination, discrimination, discernment, or perception. For example, after the eye faculty contacts a visible form, the visual consciousness and mental consciousness discern that form. If a state of joy or distress arises, or if one abides in a neutral state of perception devoid of joy or distress, three kinds of feelings emerge. This constitutes a triad of the faculty, object, and consciousness — one set for the three realms (of feeling). The other five faculties — ear, nose, tongue, body, and mind — function similarly to the eye. When a faculty contacts its object, consciousness arises, and the mind manifests perception, producing one of three types of mental states: joy, distress, or neither joy nor aversion (equanimity). These constitute five sets of fifteen dhātus (realms). Combined, they form six sets of eighteen dhātus. After each faculty contacts its object, there arise three dhātus: faculty, object, and consciousness. The six faculties contacting the six objects together constitute the eighteen dhātus, each giving rise to the three feelings — pleasant, unpleasant, and neutral. These are the eighteen objects of mental observation.