Duality is the existence of opposing forces in the universe. Matter versus anti-matter, right brain versus left brain, and day versus night are a few examples. Some philosophers believe that duality is ultimately an illusion, and instead everything and everyone are manifestations of the same unified energy.
This principle is evident in almost every aspect of our universe, from the smallest particles in quantum physics, to complex and intangible things like human personality and interaction.
From a paradigm of natural science, almost everything is based on some sort of rhythm, or oscillation between one extreme and another. For example, the earth rotates to produce the rhythm of the sun rising and falling, or day and night. As the earth travels around the sun, we experience the rhythm of the seasons. Human beings and some animals oscillate consciousness between awake and asleep. Most animals have a male and female versions. There is a cycle or rhythm of life as new life is born and goes through the process of aging. Even stars and galaxies have a life cycle as new stars are born and old ones die in spectacular super-nova explosions.
From the paradigm of quantum physics, experiments to discover the nature of light and energy have produced contradictory results. Sometimes energy behaves as a particle, and sometimes it behaves as a wave. The truth about energy is not yet known, but a common perception of energy is that it either has a dual nature, or has a counterpart that represents its opposite.
From the paradigm of sociology, people tend to divide themselves into opposites: republican or democrat, business-like or creative, conformists or anti-conformist, religious or scientific, logical or emotional, capitalist or socialist, or masculine or feminine, rich or poor, criminal or upstanding citizen, just to name a few. Interestingly enough, each side believes wholeheartedly that they are right.
From the paradigm of psychology, our emotions oscillate between happy and sad, we have both creative and logical ways of thinking (although we may prefer one over the other), we form our personalities and competencies through both nature and nurture, we might make decision based on either emotions or logic, or we might struggle with feelings of both confidence and doubt.
From the paradigm of philosophy, duality is essential for existence. Life cannot exist without death, nor good without bad, nor male without female. Ultimately we cannot even know who we are unless we know who we are not.
The thing to take away from the principle of duality is that there are different ways of thinking, and not to get stuck, thinking you are right, in one way without considering the other. The first step is to realize that there is another side. The next step is to use critical thinking to decide what you believe. By truly internalizing all aspects of the issue, and realizing that each side is “right” from their perspective, you will find your place not somewhere in between, but above.