There is some wisdom to be found in understanding the four classical elements: earth, water, air and fire. They may seem a little dated these days when we have classified many elements in the periodic table and we know that water is made of hydrogen and oxygen, and that air contains a number of different elements like nitrogen, oxygen and argon. But there is a simple wisdom in the understanding of the four elements.
You see, the four elements are the expression of the reality that underlies the universe. Earth symbolises stability, unmoving, unchanging solidity. Water symbolises fluidity and continual change. Air symbolises boundlessness, transparency, clarity, being imperceptible but essentially present. Fire symbolises energy, vibration, expression and power. Even from a basic perspective we can see the progression from fire to air to water to earth. It shows the expressive force of fire contracting to air then water then earth. There is a range or spectrum of energy, from the most energetic to the least, from the most unstable to the most stable.
What's this got to do with Non-Duality? It's a way of understanding how the true non-dual nature of Reality is expressed in the appearance of the world. What that means is that the true non-dual nature of Reality is unseen, indivisible, unmoving, unchanging life energy. It's expressed in and through the world in the apparent separation of a Singularity into a spectrum of elements that range from near to far, from high to low, from light to dark, from fire to earth, from life to death. The four elements are the play of the Singularity expressed as an explosion and condensing of frequencies of vibration. In other words, the world of energy, gases, liquids and solids, is an expression of a Singular Reality as if blown apart. High energy is close to the heat, light and source of Life. Low energy becomes cold, distant and dark. Once apparently separated they can then interact as though they are different elements.
But even the most cold and distant of solid forms is not truly impermeably solid. There is only an appearance of solidity. Reality is a Singularity. It doesn't really get blown apart into a range of frequencies. That's just how it appears. The Reality is that there can never be anything other than the Singularity. The four elements are a play of separation that form the building blocks of the world, whilst the world remains timelessly as the Singularity.
The Singularity is the Quintessence. It seems to be the centre of the cross of the four elements, the place from which they have come, the source which separates outwards to form the elements. But truly the Quintessence is timelessly the whole of the elements, without separation. Picture a cross and consider each arm of the cross to be one of the four elements. It's not that the centre is the Quintessence. It is more like the Quintessence is the whole of the cross. The cross is truly undivided. The four elements are never truly separated. They only give the appearance of separation whilst remaining timelessly Whole, eternally One.