Dualism

Dualism states that there are two different states with coexistence: One cannot be without the other, and they define each other.

In our world we have many different beliefs that base upon this: The principle of Yin and Yang, or the fight between Good and Evil for the majority of religions.

I personally take Yin and Yang as the representative, because the idea behind it is that the universe bases on these two principles. Associations to it are typically: Male and female, active and passive, motion and stillness.

But it is not being divided into two extremes, no. While in the dark is a dot of white in the white there is a dot of black as well. The symbol for it means that while they are opposites they are still connected. They complement and form the whole, defining each other.

Tai Chi is trying to use that, it uses the balance between the two to achieve transmutation freely between the two.