We eat and drink.
We don't know where it comes from.
We defecate and urinate.
We don't know where it goes to.
There is something out there: a void with which we live in perfect harmony.
Knowable only by the mind.
We cannot know what is going to happen in the next moment, yet our very existence depends on assumptions about it. Experience is nothing but memory, but it enables the global community to develop. However the fact remains, we do not know the imminently arising moment.
Unknowingly, we correctly assume that we don't know, whilst incorrectly thinking that we do, and thus unknowingly make efforts to arrange our knowledge base so as to manage selected projections in line with expectation. This very effort is a disturbance of the heart's essential pure state, is at variance with the way things are, and thus is a source of conflict. In the normal course of practice, we may well experience an occasional, brief respite from this stress, and be suitably impressed with that result, but the root cause could easily escape our awareness for eons to come.
At this level Dukkha is a subtle boredom: a dissatisfaction resulting from assumptions that the immediate future is knowable. This is enough to keep us separated from the transcendent quality of the present moment.
There is a need to investigate this not knowing, that we don't know about, by carefully focusing on it's resulting stress: Dukkha. Only when it is perfectly clear that we do not and cannot know, will we skilfully find the essence of the "here and now": that limitless eternity of the present moment.
When the Citta is completely free from greed and hatred, all that remains of the Psyche - personification of the currently running process - is identification. Because this is free from greed and hatred, it's relative purity becomes it's next hurdle: it is utterly seduced by it's own reflections and like Narcissus, drowns in them. For too many of us, this is as far as it goes: we become famous teachers.
However, for those that know of the way, this state of affairs degenerates into conflict with the way things actually are, in and of themselves: Dukkha - stress, the practitioner gets back to working with the knowable.
The Four Primary Elements are a way of analysing bodily sensations, to distinguish them from mental feelings. And also as a description of energy - bodily and mental.
Einstein's formula for energy-mass equivalence was borrowed and brought here as a metaphor for interchangeable usage when viewing subjective sensations. The choice of terminology being at the discretion of the practitioner: whatever gave rise to the clearest comprehension at any one time.
The Earth element is pressure, whether solid or space, [there being enough energy in one cubic centimetre of vacuum to power the city of New York for one year: zero point energy]; the Air element is vibration or movement; the Fire element is heat, temperature, combustion or digestion.
Water is a holding energy, spatially binding the other three elements together, so as to maintain the perceived object's form. Whilst supporting or enabling the existence of the other three elements, it is only inferred, or clearly comprehended. It may be regarded as the medium within which the other three elements serve as it's constituent characteristics.
Thus water is understood as the primal requisite for energetic existence [life], though, in and of itself, is entirely undetectable. However, at the disappearance of the water element, the other three are said to revert to their primal condition, as in the dissolution of the body at death.
Feelings result from contact between consciousness and objects of the six sense bases, also the other four Cundhas: all of which are bodily sensations of one kind or another. Clarity around this gives way to Anatta voidness.
Similarly, seeing changing Dukkha with sufficient clarity gives way to Anatta voidness.
What does it mean? Object to be taken fairly literally: may be investigated, but the point is all within the second question.
Whom does it serve? Try to find the subject that benefits from this knowledge. Investigate what ever shows up. Don't get concerned with further questions about who is it that knows "that", keep it all down to just the two questions, keep it simple!
The four Aasavas, Views and Ignorance, Sense Desire and Eternal Existence. The first pair may be seen as two sides of the same coin, as also the second pair similarly for a second coin. Then again, both coins could be seen as a further two sides of a more encompassing coin: Outflows.
"Not knowing" and "Don't Know" are merely misnomers for the effort-to-know within the mind of the unenlightened practitioner. Only in the mind of the Arahat is this the correct designation. For the rest of us it is a remaining activity that has yet to become clearly comprehended.