Realism is one of the dominant schools of thought in international relations theory, theoretically formalising the Realpolitik statesmanship of early modern Europe. Although a highly diverse body of thought, it is unified by the belief that world politics is always and necessarily a field of conflict among actors pursuing wealth and power. The theories of realism are contrasted by the cooperative ideals of liberalism in international relations. Realists are divided into three classes based on their view of the essential causes of interstate conflict. Classical realists believe it follows from human nature; neorealists attribute it to the dynamics of the anarchic state system; neoclassical realists believe it results from both, in combination with domestic politics. Neorealists are also divided between defensive and offensive realism. Realists trace the history of their ideas back through classical antiquity, beginning with Thucydides. Realism entails a spectrum of ideas, which tend to revolve around several central propositions, such as: State-centrism: states are the central actors in international politics, rather than leaders or international organizations; Anarchy: the international political system is anarchic, as there is no supranational authority to enforce rules; Rationality and/or egoism: states act in their rational self-interest within the international system; and Power: states desire power to ensure self-preservation. Realism is often associated with realpolitik, as both deal with the pursuit, possession, and application of power. Realpolitik, however, is an older prescriptive guideline limited to policy-making, while realism is a wider theoretical and methodological paradigm to describe, explain, and predict events in international relations. As an academic pursuit, realism is not necessarily tied to ideology; it does not favor any particular moral philosophy, nor does it consider ideology to be a major factor in the behavior of nations. However, realists are generally critical of liberal foreign policy. Priorities of realists have been described as Machiavellian, single-mindedly seeking the power of one's own nation over others, although realists also advocated the idea that powerful nations concede spheres of influence to other powerful nations. Common assumptions The four propositions of realism are as follows. State-centrism: States are the most important actors. Anarchy: The international system is anarchic. No actor exists above states, capable of regulating their interactions; states must arrive at relations with other states on their own, rather than it being dictated to them by some higher controlling entity. The international system exists in a state of constant antagonism (anarchy). Egoism: All states within