The four elements - fire, water, earth and air. Reincarnation. As we all know, these are important aspects of Avatar. All of these aspects are derived from the brilliant scholars of Ancient China and reincarnation in particular has become an integral part of modern philosophies. Surely, it is not possible for any other country to come up with such an impressive (yet, in the case of elements, false) theory.
Well, that is not the case. Another country had similar ideas, the cradle of Western Civilisation itself, the birthplace of Democracy, philosophy and history. That country is Greece.
Before the Greeks discovered that things such as metal and hydrogen were the true elements, they believed in the elements as seen in Avatar. The Greeks believed in four elements, γῆ (pronounced yi), ὕδωρ (ither), ἀήρ (air) and πῦρ (pyr); known respectively as earth, water, air and fire. The famous Aristotle, teacher of Alexander the Great, believed there to be four qualities, each element was attributed two. The qualities were wet, dry, hot and cold (see Figure 1). This would put fire and water as opposites and air and earth as opposites. Aristotle also believed in a fifth element named αἰθήρ, pronounced as aether. He described it as the pure element, from which all stars are made.
Another philosopher, proclus believed in three qualities - sharp/blunt, subtle/dense and mobile and immobile (see Figure 2). According to his model, fire and earth were opposite, with air and water being intermediate, perhaps you could refer to them as transition elements. His theory was not very popular in comparison to that of Aristotle, which appears to agree more with the Avatar model of fire against water and air against earth.
The Greeks took their study of the elements even further and many hypothesised that there is one element which is the root of all others. This hypothesis is used in my fanon, the Polybender Saga and is the basis of elemental conversion. Thales believed that the earth was a disk of rock floating on water and that the sky was air that condensed into water for rain, thus, he concluded that water was the original element. Heraclitus said it was fire and Anaximenes believed it to be air. All of them believed that by adjusting the density of the element, you could change it.
Oddly enough, a famous Greek mathematician - Pythagoras believed in reincarnation. He believed that when people died, they would reincarnate into beans and progress into higher forms of life until they finally reached the ultimate form - a mathematician. Pythagoras and his followers were perhaps the only westerners to believe in reincarnation, rather than an afterlife in Hades, Heaven or what have you.
Avatar was based on Chinese philosophies, but could just have easily been based on the Greek. Perhaps we would have Zeus instead of the spirits.