One thing that you should always keep in mind is that no element can truly florish without the others.
A mere look at the images I use on this site shows that they are actually never standing alone. The element of Water, for example, depicted as an ocean with waves and foam - without Air there would be none of that, without Air the water would be a still, unmoved plain. It would not even have a colour.
The Air, on the other hand, depicted as a sky with clouds - what are the clouds made from? Where do they come from? There would be no clouds without water. There would be no clouds without any warmth in the atmosphere coming from Fire. What is the mind without the spark of inspiration, without the depth of emotion? And where would all the Air be if there wouldn't be an Earth?
The Earth, shown with green plants - it's nothing new that plants need Water, Air and Warmth to grow. Without the other elements, the Earth would be dead. In the spiritual sense, Earth alone is dumb, dull, dead and cold.
Now the element of Fire, which seemingly does not need the others to exist. Yet without Earth giving a base and the fuel to Fire the flames would be short-lived, without Air they couldn't fly. And at least in the spiritual sense, the Fire even needs Water or else its flaming spirituality would be without depth, without emotion, nothing but an empty, meaningless combustion.
Spiritual Water needs Fire as well, to add warmth to its bottomless depths, to provide the spark of animation to its passive placidity. The spirit of the Air is needed to add sense and mind to the primeval emotion to bring it from the unconscious to the conscious. And all that would be in vain wouldn't there be the Earth on which everything could be grounded.
So before you rejoice when a layout of cards consists of one or two elements only because that is so much easier and there won't be any disturbing intruders to complicate matters or 'ill-dignify' a beautiful card, think twice. True harmony only lies in a good balance, and a good balance requires all the participiants.
When we look at the Greek characteristica of the four elements - cool, warm, moist and dry - we'll see that Fire is considered warm and dry, Water cool and moist, Air is moist and warm and Earth dry and cool. In each case the first attribute is the dominant one while the second reflects the attribute of another element.
Dry and moist represent form versus flexibility, while warm and cool represent separation versus union. The dry power forms stable structures while the moist power is the opposite, evading every form and floating about and around. The warm power separates and dissolves, while the cold power unites and binds things together.
As mentioned in 'Basics' Yechidah is what is generally called the fifth level of the soul, or rather the infinite, incromprehensable something in which the soul exists. It is Yechidah where all other levels come from. We might call it the Infinite, the ultimate Spirit or the Aether in the sense of Aristotle who viewed it as the fifth element (quinta essentia) and called it 'idea'. The 'idea' had its revival in early physics when it was assumed that there had to be some 'space-filling substance' as a transmitter medium for all kinds of energies.
Still Einstein said: "More careful reflection teaches us, however, that the special theory of relativity does not compel us to deny aether. We may assume the existence of an aether; only we must give up ascribing a definite state of motion to it, i.e. we must by abstraction take from it the last mechanical characteristic which Lorentz had still left it."
In short, Yechidah - the Infinite, the Universe, the All, the Spirit, the Divine, the what-ever-you-call-it - is the something where everything is in and comes from. Now that is clear, isn't it?
If we take the circle of the elements from the top of this page and place it in Yechidah, it would look something like the image aside.
While the picture is certainly flawed - never mix up Yechidah with Fire - it is my best effort to symbolize the subject. Apart from that it is a bit complicated to produce 'pure brilliance' in PhotoShop... ;)
It is also supposed to show the special status of the Earth which is not part of the elements bred from Yechidah but has been there all along.
We will meet Yechidah again when elaborating on the Elemental Dignities, when it comes to the Aces (belonging to Kether) and the three trumps that come from Kether (Fool, Magician and High Priestess).