Depending on what they see as the cause of the conflict between states, realists can be divided into three classes: 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.