International relations (IR) is the study of relationships among countries, the roles of sovereign states, inter-governmental organizations (IGO), international non-governmental organizations (INGO), non-governmental organizations (NGO), and multinational corporations (MNC). International relations is an academic and a public policy field, and so can be positive and normative, because it analyzes and formulates the foreign policy of a given State. As political activity, international relations dates from the time of the Greek historian Thucydides (ca. 460–395 BC), and, in the early 20th century, became a discrete academic field (No. 5901 in the 4-digit UNESCO Nomenclature) within political science. However, international relations is an interdisciplinary field of study.