International relations theory is the study of international relations from a theoretical perspective; it attempts to provide a conceptual framework upon which international relations can be analyzed. Ole Holsti describes international relations theories as acting like pairs of coloured sunglasses that allow the wearer to see only salient events relevant to the theory; e.g. an adherent of realism may completely disregard an event that a constructivist might pounce upon as crucial, and vice versa. The three most popular theories are realism, liberalism and constructivism.