THE SIGNS.

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Yin-Yang:
Like the sign burned Water, Yin-Yang symbolizes the union of opposites.
The opponents are: Light and Darkness, Heaven and the underworld, Men and Women, Creation and Destruction Day and Night...
We must take into account that in the entire universe there is a constant duality, is that there are things that are inconsistent, but sometimes it is necessary that both unite and so that may exist and creating harmony. For example, so that someone can come to this world needs is a man and a woman, because otherwise both would die early in isolation, therefore we speak of a duality of creation. The same goes for the day and night, because we need the night to rest and be able to do something the next day.
There are other cases that are a little complicated to understand, such as Life and Death, in this case, sometimes it is necessary for something to die so that something new is born, or someone dies to revive in another plane of existence, another case: Sometimes, we need both the bright side, as the dark side, which means that sometimes we must be firm despite the fact that it appears that we are evil, for instance, when someone scolds (Without violence) to its son because they did something wrong, you may even feel bad about it, but something is needed.