Copyright is an idea which developed after the development of print, which allowed for mass copying.
In the United States, the term of copyright, which is expressly limited by the Constitution, has been regularly extended by Congress, most recently by the Sonny Bono Copyright Term Extension Act to life of the author plus 70 years. This act is challenged by the Supreme Court case Eldred v. Ashcroft.
The extent of the legal consequences of copyright has similarly been extended in recent years by laws such as the Digital Millenium Copyright Act prohibits production and dissemination of technology that can circumvent measures taken to protect copyright, not merely infringement of copyright itself, and heightens the penalties for copyright infringement on the Internet.
The idea of copyright did not exist in ancient times, when authors frequently copied other authors at length in works of non-fiction. This practice was useful, and is the only way many authors' works have survived even in part. The copyright system was created expressly for the purpose of encouraging authorship. In the domain for which it was invented--books, which could be copied automatically only on a printing press--it did little harm, and did not obstruct most of the individuals who read the books.
The case of programs is very different from that of books a hundred years ago. The fact that the easiest way to copy a program is from one neighbor to another, the fact that a program has both source code and object code which are distinct, and the fact that a program is used rather than read and enjoyed, combine to create a situation in which a person who enforces a copyright is harming society as a whole both materially and spiritually; in which a person should not do so regardless of whether the law enables him to. --GNU Manifesto
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