Found the book "The Royal Diaries: Sondok, Princess of the Moon and Stars: Korea, A.D 595.
Found on: Page 6, on the entry of "16th day of the 11th moon, 16th year of King Chinp'young".
It mention thats in our universe there are to forces. The force that represents dark, negative and feminine things in the universe is called Um (chinse call it Ying). Then there is a force that represents the light, positive and masculine things in life. For things to function they need to be in balance.
They cant live without the other.
" There is no postive force without the negative force just as there is no day without night".