It is the general idea put forward by Proudhon in 1840 that unites him with the later anarchists, with Bakunin and Kropotkin, and also with certain earlier and later thinkers, such as Godwin, Stirner, and Tolstoy, who evolved anti-governmental systems without accepting the name of anarchy; and it is in this sense that I shall treat anarchism, despite its many variations: as a system of social thought, aiming at fundamental changes in the structure of society and particularly — for this is the common element uniting all its forms — at the replacement of the authoritarian state by some form of non-governmental cooperation between free individuals.

Prologue - Anarchism : A History of Libertarian Ideas and Movements (1962)