Changes

Making the GPL perpetuate itself is rather tricky.  Normally, when one purchases or downloads software, one legally accepts the license, promising to abide by it in perpetuity.  This is often done by clicking on an "End User License Agreement" (EULA) during the installation process.  But source files have no license that they can demand that one accept.  So the trick is to use the ''copyright'', which is a legal principle that is generally accepted by society.  But merely placing a normal copyright notice at the top of each source file doesn't really prevent someone from modifying the file and giving it to other parties, having it wind up with a license of one's own choosing.

Making the GPL perpetuate itself is rather tricky.  Normally, when one purchases or downloads software, one legally accepts the license, promising to abide by it in perpetuity.  This is often done by clicking on an "End User License Agreement" (EULA) during the installation process.  But source files have no license that they can demand that one accept.  So the trick is to use the ''copyright'', which is a legal principle that is generally accepted by society.  But merely placing a normal copyright notice at the top of each source file doesn't really prevent someone from modifying the file and giving it to other parties, having it wind up with a license of one's own choosing.

The Free Software Foundation uses what it calls a "copyleft" (a play on the word "copyright").  This is basically a normal copyright notice but with added verbiage along the lines of "You must obey the GPL".  Whether you have "accepted" the license or not, copyright law does not permit you to do ''anything'' with the file (other than read it) without obeying the copyright notice.  If that notice says you must obey the GPL, you are not allowed to do anything with the file except in accordance with the GPL. And the GPL provides that any modified and redistributed file must bear the same "copyleft" and licensing terms.  That way, the GPL perpetuates itself and achieves the goal of having software developed from each according to ability and used according to need.<ref>http://blog.lab49.com/archives/659</ref>

The Free Software Foundation uses what it calls a "copyleft" (a play on the word "copyright").  This is basically a normal copyright notice but with added verbiage along the lines of "You must obey the GPL".  Whether you have "accepted" the license or not, copyright law does not permit you to do ''anything'' with the file (other than read it) without obeying the copyright notice.  If that notice says you must obey the GPL, you are not allowed to do anything with the file except in accordance with the GPL. And the GPL provides that any modified and redistributed file must bear the same "copyleft" and licensing terms.  That way, the GPL perpetuates itself and achieves the goal of having software able to be developed by anyone.

 

Some people have suggested that this smacks of Communism or Socialism ("From each according to his ability, to each according to his need.")  But the economic issues involved in the Socialism-vs.-Capitalism debate ("state ownership of the means of production") do not apply to information.  Information is essentially free. State ownership of the means of production is meaningless when the means of production consists of clicking "download".

Copyleft as a mechanism requires that any software which uses code from copylefted software to be licensed under the same terms—making it copylefted as well. In the long run, this will lead to more and more software being licensed under copyleft terms. Not only that, but, due to the nature of copyleft, a lot less diversity in licenses is possible—in fact, the most popular copyleft license, the GNU General Public License (GPL), is used for the majority of copylefted software. Some people have held that, if this tendency is not stopped, all open source software will soon be licensed under one license.  These fears are exaggerated.  There are quite a number of "free" software licenses, such as OSI and Creative Commons Licenses, which still legally enforce the open source principle.

Copyleft as a mechanism requires that any software which uses code from copylefted software to be licensed under the same terms—making it copylefted as well. In the long run, this will lead to more and more software being licensed under copyleft terms. Not only that, but, due to the nature of copyleft, a lot less diversity in licenses is possible—in fact, the most popular copyleft license, the GNU General Public License (GPL), is used for the majority of copylefted software. Some people have held that, if this tendency is not stopped, all open source software will soon be licensed under one license.  These fears are exaggerated.  There are quite a number of "free" software licenses, such as OSI and Creative Commons Licenses, which still legally enforce the open source principle.