What is mental phenomenon?

Mental formation (sankhara) is the current object of awareness of mind consciousness.

Mind is able to perceive in direct (things as they are), representative (deluded, distinguished and prejudiced) and purely in mind-object way (abstract, not in touch with reality).

• The list of 51 mental formations: https://plumvillage.org/transcriptions/51-mental-formation/

• The Four Layers of Consciousness from the perspective of Yogacara: https://www.lionsroar.com/the-four-layers-of-consciousness/

Edit: Apologies. Edited so it rather displays all the relevant text rather me trying to give a summary.

Presence of mental formation/phenomena:

Your ear and the music come together and provoke the manifestation of the mental formation called touch, which causes store consciousness to vibrate. That information, a new seed, falls into the store continuum. Store consciousness has the capacity to receive the seed and store it in its heart. Store consciousness preserves all the information it receives. But the function of store consciousness isn’t just to receive and store these seeds; its job is also to process this information.

Absence of mental formation/phenomena:

There are times when sense consciousness operates in collaboration with store consciousness without going through the mind. It’s funny, but it happens very, very often. When you drive your car, you are able to avoid many accidents, even if your mind consciousness is thinking of other things. You may not even be thinking of driving at all. And yet, most of the time at least, you don’t get into an accident. This is because the impressions and images provided by eye consciousness are received by store consciousness, and decisions are made without ever going through mind consciousness. When someone suddenly holds something close to your eyes—for instance, if someone is about to hit you, or when something is about to fall on you—you react quickly. That quick reaction, that decision, is not made by mind consciousness. If you have to make a quick maneuver, it’s not your mind consciousness that does it. We don’t think, “Oh, there is an accident, therefore I have to quickly swerve to the right.” That instinct of self-defense comes from store consciousness.

Phenomena source is one of the twelve sources. It is apprehended only by mental consciousnesses: In Commentary to the Middle-Length Lamrim, Geshe Jampa Gyatso says:

The twelve sources are the six object-possessors (the eye source, ear source, nose source, tongue source, body source, and mental source) and the six objects (form source, sound source, odor source, taste source, tangible object source, and phenomena source).

There are two types of mental consciousnesses:

1. Conceptual consciousnesses
2. Non-conceptual mental consciousnesses

An example of a conceptual consciousness is a memory-consciousness. Examples of non-conceptual mental consciousnesses are the five clairvoyances, and dream-consciousnesses. The five clairvoyances are: minds apprehending minds in the continuum of other persons, the divine eye, and so forth.

If it is a phenomena source, it cannot be apprehend by a sense consciousness. However, if it is a mental consciousness, it does not necessarily apprehend phenomena source. For example, the divine eye apprehends actual form (i.e. form which is form source).

Furthermore, an example of a mental consciousness apprehending phenomena source is a dream-consciousness. A dream-car is called 'form that is not form source but phenomena source', it is not fully qualified form. In Commentary to Lorig, Geshe Tenzin Tenphel says:

The blue of a dream is a phenomena source, an object of mental consciousness, whereas actual blue is a form source, an object of eye consciousness.

An example of a valid conceptual consciousness is a memory-consciousness remembering one's mother. Its appearing object is a mental image, and the actual mother is the object of engagement. A mental image is a phenomena source, and therefore a conceptual consciousness apprehends its object of engagement by way of a phenomena source appearing.

According to Asanga and Vasubandhu's Abhidharma texts, here are five examples of phenomena source:

1. Dream-objects
2. Kasinas (such as the water or earth kasina to the perspective of a yogi)
3. Appearances of hair falling, etc. appearing to an eye that is subject to disease.
4. The appearance of two moons when there is one moon in actuality.
5. The appearance of an actual face when we look at the reflection of a face in the mirror.

Mental phenomenon in the context of the six senses is things like: thought, memories, mental images... things like that. Mental formations also fall in this category, these are basically thoughts.

The arising mind consciousness is thus aware of thoughts, or of memories and so on.

Taste is just taste. When you differ, you assign different signs/characteristics. This is a mental proces after receiving the basic taste object. As soon as you differentiate you are passed the point of 'let tasting just be tasting', imho. That's why on a retreat you can, at a certain point, no longer differentiate let alone know what you're eating. Concepts like salty, sweet or bitter no longer come up. Well, at least that's my experience.