So in my poem yesterday, I used the lines

"Do you see light or only dark

or realize that one cannot exist without another"

as all of carrying on the overall theme of the poem of contradicting aspects of a person or people, yin and yang, if you will, and posing whether the reader can perceive the contradictions or can only deal with seeing oneself as binary, and whether the person is a black and white thinker or capable of seeing nuance (grey) and also to mean that if only one thing exists then one can't know what that thing is because there is no contrasting aspect. Light dark, good evil, love hate, pleasure pain, happy sad, etc etc. To know one, and everything in between, both must exist.