Among the Six Dusts, the Speed of Propagation of the Form Dust Is Fast.

Question: Last night while meditating and observing the breath, I suddenly felt a flash of white light pass through my head, and immediately afterward I heard the sound of a stool being moved upstairs. Is the flash of white light in the head something produced from the contact of faculty and object? Answer: The sound of the stool being moved upstairs travels to the supreme faculty (indriya) in the brain; when faculty and object make contact, consciousness is generated, and the ear consciousness hears the sound. Before the sound is heard, the sound object suddenly makes contact with the supreme faculty, and the supreme faculty has an abnormal reaction—the flash of white light in the head is that reaction. Because the five supreme faculties are also material form (rūpa) composed of the four great elements, and the sound object is also material form composed of the four great elements, if a relatively large sound object travels to the supreme faculty, it will impinge upon the supreme faculty of the body faculty. Upon receiving this sudden impingement, the brain of the body faculty has a stress reaction.  

If one is in relatively quiet meditative concentration (samādhi), with the mind unprepared, and suddenly an object-realm appears, the manas will prompt the body faculty to have an abnormal reaction. If one is in the usual scattered state, such a sudden stimulus reaction will not occur. Another example: a very loud sound such as thunder can also vibrate the eardrum, even causing deafness in the ear faculty. The eardrum belongs to the physical body of the body faculty, and vibration belongs to the object of touch, which is discerned by the body consciousness. This shows that the sound object has material properties and is also material form (rūpa); therefore a portion of material form can also be discerned through the eye consciousness, and the flash of white light in the head is precisely what is discerned by the eye consciousness. In the future, when the six faculties interpenetrate and function interchangeably, hearing sounds will not require the ear consciousness, seeing forms will not require the eye consciousness, smelling scents will not require the nose consciousness, tasting flavors will not require the tongue consciousness, feeling tangibles will not require the body consciousness, and discerning dharmas will not require the mind consciousness.  

For object-realms originating from the same source, does the eye consciousness see them first, or does the ear consciousness hear them first, or does the nose consciousness smell them first, or does the tongue consciousness taste them first, or does the body consciousness touch them first? The speeds at which the six object-realms propagate are certainly different; exactly which one propagates fastest still requires much observation. For example, when thunder occurs in the sky, does the eye consciousness first see the lightning, or does the ear consciousness first hear the thunder? Generally speaking, the propagation speed of form objects is faster than that of sound objects; for object-realms transmitted from the same point of origin, the eye consciousness sees them first, and the ear consciousness sees them later. Another example: when sunlight travels over, the eye consciousness first sees the light, and the body consciousness afterward feels the warmth.