Anarchism is a political philosophy which considers the state undesirable, unnecessary and harmful, and which promotes instead a stateless society, or anarchy. It seeks, more broadly, to diminish or even abolish the authority principle in the conduct of human relations. Individual anarchists may have additional criteria for what they conceive to be anarchism, over which there can be wide disagreement. The Oxford Companion to Philosophy says, "there is no single defining position that all anarchists hold, and those considered anarchists at best share a certain family resemblance."
There are many types and traditions of anarchism, not all of which are mutually exclusive. Strains of anarchism have been divided into the categories of social and individualist anarchism or similar dual classifications. Anarchism is often considered to be a radical left-wing ideology, and much of anarchist economics and anarchist legal philosophy reflect anti-statist interpretations of communism, collectivism, syndicalism or participatory economics; however, anarchism has always included an individualist strain, with that strain supporting a market economy and private property, or unrestrained egoism that bases right on might.
Others, such as panarchists and anarchists without adjectives, neither advocate nor object to any particular form of organization as long as it is not compulsory. Some anarchist schools of thought differ fundamentally, supporting anything from extreme individualism to complete collectivism. The central tendency of anarchism as a social movement have been represented by communist anarchism, with individualist anarchism being primarily a philosophical or literary phenomenon. Some anarchists fundamentally oppose all forms of aggression, supporting self-defense or non-violence, while others have supported the use of some coercive measures, including violent revolution and terrorism, on the path to an anarchist society.