From Richard Stallman’s “Transcript of Richard Stallman at the 4th international GPLv3 conference; 23rd August 2006” (FSF Europe: 23 August 2006):
Specifically, this refers to four essential freedoms, which are the definition of Free Software.
Freedom zero is the freedom to run the program, as you wish, for any purpose.
Freedom one is the freedom to study the source code and then change it so that it does what you wish.
Freedom two is the freedom to help your neighbour, which is the freedom to distribute, including publication, copies of the program to others when you wish.
Freedom three is the freedom to help build your community, which is the freedom to distribute, including publication, your modified versions, when you wish.
These four freedoms make it possible for users to live an upright, ethical life as a member of a community and enable us individually and collectively to have control over what our software does and thus to have control over our computing.