This concept, central to realist thinking, has multiple meanings and is surrounded by controversy about its practical implications. It generally refers to an equilibrium of power whereby the capabilities of one state or combination of state are balanced and checked by the capabilities of others. Some argue that achieving such an equilibrium is difficult, and can be attained only by careful diplomacy. Others, notably structural realists, argue that the international system has an inherent tendency towards equilibrium. Hobbes, whose influential ‘state of war’ image was outlined earlier, believed that actually existing international relations could and often were ameliorated by policies of prudence and expediency that created an equilibrium of power.