"Every explicit duality is an implicit unity."

Fire and water are seen as two polarities which create electricity in universe. And electricity creates all matter. And all matter is made from atoms. So, everywhere where we have atoms, there are these two polarities: Water and Fire. The natural forces of fire and water, which evidently exclude each other, are brought together in a unity of opposites. One is radiating, one is compressing, one positive and other negative. One can't exist without other. What is explicitly two can at the same time be implicitly one. Like black and white are inseparable. 2 curves of a single sine wave. Any experience that we have through our senses, whether of sound, or of light, or of touch, is a vibration. And a vibration has two aspects: one called "on", and the other called "off". Vibration seems to be propagated in waves, and every wave system has crests and it has troughs.

Both forces, fire and water, are purifying as well as protective and are viewed by many as being connected with the cosmic powers of the sun and moon.

Fire is one of the four classical elements in ancient Greek philosophy and science. Fire is considered to be both hot and dry and according to Plato, is associated with the Tetrahedron. It's symbol is triangle. It was commonly associated with the qualities of energy, assertiveness, and passion. In Vedic tradition Agni is a deity. Agni is one of the most important of the Vedic gods. He is the god of fire and the acceptor of sacrifices. The sacrifices made to Agni go to the deities because Agni is a messenger from and to the other gods. He is ever-young, because the fire is re-lit every day, yet he is also immortal. Fire is also linked to Surya or the Sun and Mangala or Mars, and with the south-east direction.

Water, like fire, is also one of the classical elements in ancient Greek philosophy and in Vedic tradition part of Panchamahabhuta or Five Great Elements and in the Chinese cosmological and physiological system, part of Wu Xing or Five Phases. Water is considered to be moist and cold. It's symbol is inverted triangle. In contemporary esoteric traditions, it is commonly associated with the qualities of emotion and intuition. In Vedic tradition Ap (áp-) is the sanskrit term for the element of water. It is also the name of the deva Varuna, a personification of water, one of the Vasus (attendant deities of Indra, and later Vishnu) in most later Puranic lists. The element water is also associated with Chandra, also known as Soma, the Moon god or the moon. And Shukra, planet Venus, who represent feelings, intuition and imagination.

"It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do."

As hinted at by the word "dual" within it, duality refers to having two parts, often with opposite and contrasting meanings, like the duality of positive and negative or up and down, in and out, etc.

Duality teaches us that every aspect in universe is created from a balanced interaction of opposite and competing forces. Opposite is not really the right term, because these forces are not just opposites - they are complementary. They do not cancel out each other, they merely balance each other. You can't fully understand one side of the dual nature of something without comparing and fully understanding the opposing side.

"Only in love are unity and duality not in conflict."

In physics, duality is the property of matter and electromagnetic radiation to be understood best through wave theory or particle theory - wave-particle duality. All behavior of light and matter can be explained through the use of a differential equation which represents a wave function. Current scientific theory holds that all particles exhibit a wave nature and vice versa.

"So it is said, for him who understands Heavenly joy, life is the working of Heaven; death is the transformation of things. In stillness, he and the yin share a single Virtue; in motion, he and the yang share a single flow."

Yin-Yang is a complex relational concept and powerful example of a philosophy underpinned by fire and water. The philosophy is at least 3,500 years old, discussed in the 9th century BCE text known as I Ching or Book of Changes, and influences the philosophies of Taoism and Confucianism. Yin and yang elements come in pairs, but note that yin and yang are not static or mutually exclusive terms. While the world is composed of many different, sometimes opposing, forces, these can coexist and even complement each other. Sometimes, forces opposite in nature even rely on one another to exist. The nature of Yin-Yang lies in the interchange and interplay of the two components. The alternation of day and night is just such an example: there cannot be a shadow without light. Yang is associated with fire, masculine, positive, whereas yin is associated with water, feminine, negative. The meaning of yin and yang is that the universe is governed by a cosmic duality, sets of two opposing and complementing principles or cosmic energies that can be observed in nature.

The origin of the yin-yang symbol is found in the ancient Chinese time-keeping system of using a pole to measure the changing lengths of shadows over the solar year. Some have suggested that the yin-yang symbol closely approximates a graphical representation of the daily change of a pole's shadow length during the year. Yang begins at the winter solstice and indicates the beginning of the period when daylight dominates over darkness and thus is associated with the sun. The yin begins at the summer solstice and represents the dominance of darkness over daylight and is associated with the moon.

Yin-Yang also represents the observation of the shadow of the earth on the moon, and the record of the position of the Big Dipper constellation through the year. These observations make up the four points of the compass: the sun rises in the east and sets in the west, the direction of the shortest shadow measured is south, and at night, the pole star points north.

So in my conclusions, Yin-Yang is, among others, fundamentally connected with the annual cycle of the earth around the sun and the resulting four seasons.

Fires triangle and waters inverted triangle together form a symbol called Hexagram. This simple symbol is taking on a different dimension as the star becomes a map of opposing forces which combine to make all living things and is the symbolic epitome of the phrase "as above, so below".

Six-pointed stars have been found in cosmological diagrams in Hinduism, Buddhism, and Jainism. In India, the Hexagram is called the Shatkona or Six-pointed Star and represents the energies of Shiva and Shakti locked in a constant embrace. It represents both the male and female form, divine union of masculine and feminine, a source of all creation. The upper triangle stands for Shiva or Purusha (the supreme being, consciousness), the lower triangle for Shakti or Prakriti (mother nature, causal matter, unmanifested cosmic energy or potential matter).

In Jewish traditions this symbol is called Magen David ("Shield of David") or most generaly known as Star of David. It symbolizes the connection of both dimensions of God, Torah and Israel: the external level of the soul connects to the external expression of G‑d via studying the exoteric parts of Torah; the essence of the soul connects with G‑d’s essence through the study and application of the teachings of Kabbalah. In Christianity it is called the Star of Creation. In Islam the symbol is known in Arabic as Khātem Sulaymān (Seal of Solomon; خاتم سليمان) or Najmat Dāwūd (Star of David; نجمة داوود).

It was also used in many brewer emblems as symbol for brewing, representing "inflammable spirit" or "fire water" or alcohol. Brewers called it "Brewing Star".