Elemental:
The Four Elements

In his Tetrasomia, or Doctrine of the Four Elements, Empedocles described these elements not only as physical manifestations or material substances, but also as spiritual essences. He associated these elements with four gods and goddesses - air with Zeus, earth with Hera, fire with Hades, and water with Nestis (believed to be Persephone): 

Now hear the fourfold roots of everything:
Enlivening Hera, Hades, shining Zeus
And Nestis, moistening mortal springs with tears.

In Empedocles' philosophy, the interaction of the four elements is influenced by the relationship between the two great life energies of Love and Strife: 
 

Empedocles explained that there are two great living forces in the universe, which he called Love (Philotês) and Strife (Neikos) and assigned to Aphrodite and Ares. According to Hesiod, the Goddess Love and the God Strife, offspring of Night (Nux), were ancient deities, predating the Olympians. The original golden age was the Reign of Aphrodite, when all things were united and Love permeated the length and breadth of the well-rounded cosmic sphere. But Strife, as the River Styx surrounding the Sphere, broke its Unity, and cleaved the One into Many. It divided the four elements, which ever since combine and separate under the opposing actions of Love and Strife to produce the changing world with its manifold objects and qualities. As Heraclitus said, "Through Strife all things come into being." Empedocles said that Strife also divided the one immortal soul of Love into many individual souls, each comprising both Love and Strife in some proportion; these immortal souls are reborn time and again into mortal bodies, which are animated by mortal souls compounded from the four elements.
from Exercise for Unity by Apollonius Sophistes
http://www.cs.utk.edu/~mclennan/BA/EaU.html
 

Empedocles explains the nature of the universe through the interaction of two governing principles, Love and Strife, on four primary elements. Unlike his predecessors, Empedocles claims that there are four elements in the universe; air, fire, earth, and water. Particular and indestructible, these elements foreshadow later developments in atomic theory by philosophers such as Leucippus and Democritus. Earlier philosophers believed that the quality of matter depends on the quantity of a particular element.....

Empedocles adds a moral dimension to his argument by associating Love with good and Strife with evil. The influence of each principle waxes and wanes in a cycle of opposition that Empedocles calls "The Vortex." At the beginning of time, Love completely dominated the universe. As a consequence, the four elements were unified into a sphere and segregated according to their type-- a quarter of the sphere was water, a quarter was air, and the remaining half was divided equally by earth and fire. However, with the introduction of Strife "The Sphere" was gradually dissolved, slowly scattering the elements throughout the universe. The complete dissolution of The Sphere was achieved by the eventual predominance of Strife. However, Love began to gather strength again, causing the elements to congregate in clusters, and thereby creating life. Eventually, the elements will form a second unity, a second "Sphere," and the cycle will reset.
from Empedocles of Akragas by Jesse Weidman:
http://www.perseus.tufts.edu/GreekScience/Students/Jesse/Jesse.html
 

Empedocles' philosophy was influenced not only by Pythagoras, but also by the ancient Greek mystery traditions, which included the Orphic mysteries and the underworld cults of Hades, Hecate, Demeter, Persephone and Dionysos. In his own thinking and writing, and in works and practices of the alchemists, neoplatonists and gnostics that further developed his theories, the four elements are not only material and spiritual forces, but also facets of a human being. Their varying combinations result in different personality types. 

Since we know that Carl Jung (1), one of the founders of modern psychology, studied mystical literature and alchemy, we can easily conclude that his conceptualization of intuition, sensation, thinking and feeling as the four basic archetypes or components of personality is clearly a derivation of Empedocles' ancient theories about fire, earth, air and water. Jung focused initially on the polarities of introversion (directing one's attention inward toward thoughts, feelings and awareness) and extroversion (directing one's energy outward toward people, actions and external objects), combining each polarity with predominances in thinking, feeling, sensing and intuiting, to develop eight basic personality types. 

The four personality variables of the Meyers-Briggs test (and its offshoots, the Keirsey test and the DDLI) (2) also appear to be a further development of this psychological philosophy. 

Most philosophers and alchemists also believed that the four elements exhibited itself in man as four varying natures and that one was more prevalent in each individual than the other three. An individual leaned more to one particular type rather than possessing equivalent amounts of all four. Empedocles said that those who have near equal proportions of the four elements are more intelligent and have the most exact perceptions.
from The Four Elements by Charlie Higgins
http://www.mension.com/elements.htm

Aristotle explains that Moistness is the quality of fluidity or flexibility, which allows a thing to adapt to its external conditions, whereas Dryness is the quality of rigidity, which allows a thing to define its own shape and bounds. As a consequence Moist things tend to be volatile and expansive, since they can fill spaces in their surroundings, whereas Dry things are fixed and structured, since they define their own form.

Opsopaus points out the various associations made to the Four Elements by Empedocles and his Greek followers,  as well as their development in western mysticism, alchemy (3) and Jungian psychology: 

Most obviously there are the macrocosmic manifestations of the Elements, for example, the land, the sea, the sky and the sun. They are also connected with the sublunary spheres: Heaven, Earth, Abyss (the subterranean water) and Tartaros (the subterranean fire). There are also microcosmic manifestations, for example, as components of the human psyche (mental, astral, etheric and physical bodies).... The Elements also represent the stages in various processes of growth and transformation.... Finally, from the standpoint of Jung's psychology, the Elements (like the gods) are archetypes; because they are structures in the collective unconscious, they are universal (present in all people). As archetypes, they are beyond complete analysis; they can be "circumscribed but not described"; ultimately they must be experienced to be understood.
from The Ancient Greek Doctrine of the Elements byJohn Opsopaus: 
http://www.cs.utk.edu/~mclennan/BA/AGEDE/index.html
http://www.cs.utk.edu/~mclennan/BA/AGEDE/Intro.html
 

Earth, for example, is cool and dry, passive and rigid, a principle of structure and materialization. Psychologically, it is therefore predominant in persons who focus extensively upon physical reality, and who may tend toward qualities such as perseverance, inflexibility, realism and pragmatism. 

Water, however, was originally the Primordial Chaos, associated with dissolution, union and transformation, and therefore with the goddess Persephone, who rules over death and rebirth in the Underworld, and whose lessons bring both tears of grief and tears of joy. Psychologically, water is likely to be the predominant element in people who tend to be flowing, flexible, oriented toward harmony or union, and inclined toward deep feeling.