There is hot and there is cold. There is male, and there is female. There is light and there is dark. There is yin and there's yang.
Life in the physical form is manifested as duality. Life in the highest sense, is manifested as singularity. However, in order to transcend duality to reach singularity and oneness, you must first master it.
How do you master duality?
Starting with yourself. See, we all go about life defining ourselves as something. When you say you are something, you are automatically negating it's opposite. I am introverted" means I am not extroverted". I am logical/creative. I am neat/messy. I am organized/disorganized. I am punctual/tardy.
Until now, you've lived in a certain pole, and have not dared ventured to the other side. Society has probably conditioned you to believe that you are just one thing and not the other, and there is no way you can change that. The lie cannot get larger than this.
In order to understand your true nature, your true self, a path to mastering duality must be taken.