Civil society organizations began to be considered as a "sector" only in the 1970s in the United States. Amitai Etzioni pioneered the use of the expression third sector, which became common in academic and political literature. However, the concept of the non-profit sector gradually gained strength in the United States, spreading internationally based on the studies conducted by Lester Salomon, who transferred premises from the North American case to international studies. For this reason, the notion that the third sector is a subsector of the private sector still predominates in Brazilian studies. On the other hand, theoretical currents such as liberal communitarianism, the theories of cooperation, of common goods, of social capital, of the European social economy, and the Latin American solidarity economy highlight the primacy of cooperation in the resolution of collective problems and ground the associative approach of the third sector, linking it to the community rather than to the market. The research problem is: what differences exist between the philanthropic and associative approaches of the third sector and what evidence indicates that the associative approach is more appropriate for studies on civil society organizations and local self-organization? It is concluded by the relevance of the notion of third sector as a set of organizations created and maintained by the civil society, linked to the community and not to the market. The research technique is bibliographic.