Duality is defined by the observer.

Day is the opposite of night. But day and night only exist to someone on the earth.

If you had a space shuttle that orbited the sun it would always be light on one side of the craft and dark on the other side. But there is no dark side of the sun. The sun is equally bright on all sides. It is the observer that defines the differences.

The differences may exist to the observer only.
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