Harmony of opposites

‘Harmony’ is a consistent, orderly or pleasing arrangement of parts, melody and ‘opposites’ means being different in nature. “Harmony of opposites” is a philologically self contradictory statement. But isn’t it that a harmony necessarily consists two opposite principles which inspite of this opposition has some underlying unity. Let’s take the example of Light and darkness both are opposite in nature. But, still both are needed for each other’s existence. It has been scientifically proved (in the concepts of interference and diffraction) that wherever there is light, there is darkness too. Moreover, perception of one is not possible if the other ceases to exit. Similarly, if we would have not known what SADNESS is like, then HAPPINESS would never have meant anything! Also let’s take the example of music; a music is harmonious only when the beats of different frequencies are present, music of a single frequency will make the it sound too bland.
So, it is by this virtue of opposition that everything in this universe exists. W T Stace in his book ‘The Critical History of Greek Philosophy’ has written that, all things contain their own opposites within them. In this struggle and antagonism between hostile principles consists their life, their being, their very existence. At the heart of things is conflict, if there is no conflict in a thing, it would cease to exist. This idea written above was expressed by a Greek Philosopher, Heraclitus also known as the Dark Philosopher.