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The symbol Yin & Yang is so old that history does not tell us who first invented it or what it first described. We know it came out of China thousands of years ago. In modern times it is used to express opposites. Hot/cold, day/night, wet/ dry, good/evil, ect. This symbol shows that as Yin increases, Yang naturally decreases. Yet at the fullest point of Yin there is a tiny dot of Yang and vice versa. So the symbol also shows a balance between the two. I think Yin & Yang perfectly describes what we speak of when we talk about balance in Nature.