The Left Hand of Darkness by Ursula K. LeGuin

The vital though that the author tries to explain and depict in this book is a complicated idea--duality in any forms. According to the book, duality means opposite elements that complement each other that can create a balance and indivisible whole, such as light and darkness, fear and courage, coldness and warmth, female and male. 

People live in the story with duality can physically manifest and manipulate one of the elements of it. But there is a restriction of this unique ability. They can only maintain this statement for a period of time. They will get back the neutral state finally. In my opinion, this interesting fact is a metaphor to human’s desire to seek a soul mate to complete themselves.

There are two main characters in this story, Ai and Estraven. As the story develops, the freezing winter continue to hurt Ai, but Estraven is not influenced by the extreme weather. However, Ai the stronger one of them, so the story becomes more dramatic. Later in the story, both of them begin to exceedly manifest opposing characteristics as their journey continues. Apparently, their interaction shows how opposite elements react to each other as a whole, which is the key idea of duality. It is the regularity that exists in nature and the human body.