According to one of the greek mythological stories, it is supposed that the universe is made up of four elements which are fire, water, earth and air. 

Air was considered to be a component of Aether ( ether) which is nothing but the absence of the other three elements. Aristotle was the one that gave them the name “ elements”, a smallest time unit of ancient sundial (the early clock based on sun position).

From lower to higher order, Earth rises out of Water, Air is over the Earth, and the Fire which is the sun is above everything.

Nature that framed us of for elements, warring within our breasts for regiment, doth teach us all to have aspiring minds.

                      ~ Niccolo Machiavelli.

There are also other references in different cultures. The I Ching, which is  the book of Changes, consists of 64 hexa grams, each has two trigrams placed one over the other. And each of those trigrams has three lines, broken Yin( which is — — ) or unbroken Yang    (----).  As there are 8 of the trigrams, there are 64 hexagrams, which is 8x8. Ultimately, the trigrams represent the combinations of the same concepts of Fire, Air, Earth and Water. 

FIRE

The fire element is considered to be the first developed element out of all others, when the universe was initially created. It is represented as the power that gives warmth, and at the same time responsible for destruction. Fire's symbolic representation is a upwards directing triangle. 

WATER

This element is considered as a cleansing power. Dreaming, healing, purification, devotion, strength, unconditional love and change are the symbolic representation that this elemnt possesses. Fresh water is the symbol of healthy and good life, while polluted water shows bad health  obviously. The symbol for the water element is an inverted triangle.

AIR.

Representing the breath of life and cleansing power, Air is the other element. It represents communication, knowledge, thinking, creativity, harmony and travel. The symbol of the element air is an upright triangle with a horizontal line.

EARTH.

The element Earth is the symbolic representation for cleaning power. It depicts fertility, stability, solidity, security, nourishment, wisdom and dependability. The symbol for this element is a complete opposite of that of air, which is an inverted triangle with a horizontal line.

As a combination they all show a symbol of a perfect star.

As above, so below.

As within, so without.

As the universe, so the soul.