On the assumption that intellectual property rights are actual rights Stallman argues that this claim does not live to the historical intentions behind these laws, which in the case of copyright served as a censorship system, and later on, a regulatory model for the printing press that may have benefited authors incidentally, but never interfered with the freedom of average readers. Still referring to copyright, he cites legal literature such as the United States Constitution and case law to demonstrate that it is meant to be an optional and experimental bargain that temporarily trades property rights and free speech for public, not private, benefit in the form of increased artistic production and knowledge. He mentions that "if copyright were a natural right nothing could justify terminating this right after a certain period of time".