Balance

"Throughout history, every period of enlightenment has been accompanied by darkness, pushing in opposition.  Such are laws of nature and balance."  -unknown

Everything is held in balance.  Chaos and disease occurs when things are out of balance.  Consider that this concept applies to every element in the universe, including state of mind.  When applied to our consciousness, it makes sense that when we are emotionally out of balance, dis-ease transpires.  Opposing forces are continuously at play in every facet.  Just as every action has an equal and opposite reaction, every force has an equal and opposite force.
In life, both positive and negative forces are necessary for existence.  The earth is consists of polar magnetic fields, which we use as a guiding force by finding north on a compass.  Both positive and negative charges are necessary for electricity to generate energy.  Without darkness we could not have light and vice verse.