22 Pope Leo XIII also defended the worker's natural right to enter into association with his fellows. Such associations may consist either of workers alone or of workers and employers, and should be structured in a way best calculated to safeguard the workers' legitimate professional interest. And it is the natural right of the workers to work without hindrance, freely, and on their own initiative within these associations for the achievement of these ends.
Source: Mater et Magistra (Vatican.va)