Yin & Yang are the 2 aspects of vital energy. Energy is one but it has 2 polarities.They are basic principles of entire universe & function.; as two opposite forces like +ve & -ve.They are complementory to each other & balance each other but at the same time they are opposite to each other. Thousands of years before Newton proved that action & reaction are equal & opposite , the Chinese had the concept of yin & yang.
Without day there can not be night & without night there can be no day. Interdepending relation of the two is necessary for the existence of either. Nei jing says that the universe is a state of oscillation of forces of yin & yang & their changes.Because yin & yang relate to each other in harmony , balance exists in the body. Good health is a state of energy balance between yin & yang . Upset in the balance is disease.