Duality! The US democracy is based on the principle of duality. Most of the conceptions on Earth are based on that principle as well. For example, we compare good to evil. In different aspects of our life, that principle exists. In sport, we compare Leo Messi with Ronaldo. In Tennis, we choose between Nadal and Federer. In Formula 1, we choose between Hamilton and Vettel. In politics, we choose between 2 parties. It is a circle. In love, we choose between a man or a woman. Our world is built that way.