Ancient philosophers banged their heads together to come up with the ESSENTIAL elements that the universe is made up of. One thought everything boiled down to water. Water is most of the earth and makes up most of us. Another believed it was dirt, or earth. After all we return to the dirt after we die so earth must be one of the basic elements. Then there were the fire starters who believed that everything burned and that energy of fire was a core element in our makeup, once we lost our fire we were gone. Then, of course, there was the Wind faction. That invisible force that influenced everything and everyone. These four elements were fought over to the point that ALL of them came to be known as the Four Essential Elements: Earth, Wind, Fire and Water.
The question then became: "How do these four elements interact with each other?" Water breaks down earth, earth blocks wind, wind enhances fire, fire boils water, water feeds earth, earth funnels wind, wind blows out fire, fire it put out by water, etc. So began the search for the Fifth Element. The Fifth Essential (quintessential) element that explained everything. That perfect answer to the questions of elemental interaction.
Socrates tackled this problem calling the quintessential element "ether" and described it as the "pure essence where the gods lived and which they breathed" The ether had no measurable qualities (hot, cold, wet, dry, etc) but you could observe its influence on planet systems and in the lives of people. Later science disproved the ether theories in favor of gravity and the atomic.
We have lost that sense of the Fifth or Quintessential Element but we still struggle to define it. Now we seek to bring together the two essential theories in Physics: Newtonian Physics based on normal gravitational influences and Quantum Physics based on EXTREME gravitational influences like black holes and bent space. So now with two contradictory theories to explain observable interactions we are left looking for the ether again. This time it is called the "Theory of Everything" which will combine quantum mechanics with general relativity. Hawking's new book "The Grand Design" attempts to solve this with his version of ether called the "Multiverse" after his failed attempt to explain it in his earlier book on string theory.
I tend to fall back on one the best philosophical minds of the first century AD. Who referred back to those Greek philosophers when he described God as not living in temples built by hands and not needing anything from us because God gives all life and breath to men. He is never far from us and if we reach into the ether we can find him for "In HIM we live and move and have our being" or God IS the ether you are looking for. That essential element that resolves and controls all the elements.
God himself is the quintessential Fifth Element.