Authoritarianism is a form of government. Juan Linz, whose 1964 description of authoritarianism is influential, characterised authoritarian regimes as political systems by four qualities: (1) "limited, not responsible, political pluralism"; that is, constraints on political institutions and groups (such as legislatures, political parties and interest groups), (2) a basis for legitimacy based on emotion, especially the identification of the regime as a necessary evil to combat "easily recognizable societal problems" such as underdevelopment or insurgency; (3) neither "intensive nor extensive political mobilization" and constraints on the mass public (such as repressive tactics against opponents and a prohibition of anti-regime activity) and (4) "formally ill-defined" executive power, often shifting or vague.