Perpetual copyright can refer to a copyright without a finite term, or to a copyright whose finite term is perpetually extended. Perpetual copyright in the former sense is highly uncommon, as the current laws of all countries with copyright statutes set a standard limit on the duration, based either on the date of creation/publication, or on the date of the creator's death. (See List of countries' copyright lengths.) Exceptions have sometimes been made, however, for unpublished works. Usually, special legislation is required, granting a perpetual copyright to a specific work. In many countries, moral rights, which may be covered under the copyright law, can last perpetually. Copyright philosophy The basic philosophical argument employed by proponents of perpetual copyright presupposes that intellectual property ownership rights are analogous to other property rights such as those associated with material goods. Proponents such as Mark Twain and Jack Valenti have stated that owners of intellectual property should have the same perpetual right to retain and bequeath this property to their descendants that owners of material goods already possess. Jonathan Zittrain, a faculty co-director at the Berkman Center for Internet & Society, illustrated this argument using the analogy: "[It] makes no sense to imagine somebody after a certain time coming in and taking your rug or your chair and saying 'Sorry, your ownership expired.'" In an op-ed published in The New York Times, author Mark Helprin argues that it is unjust for a government to strip copyright holders of their exclusive rights after a set period of time. He also writes that copyright expiration transfers wealth from private copyright holders to corporations: Calls for perpetual copyright have been widely criticized. Lawrence Lessig organized a community response to Helprin's editorial. Public Knowledge issued a response which argued that copyright expiration ultimately provides a net benefit to society. It distinguishes intellectual property rights from those associated with material goods; the latter "are scarce and rivalrous: [they] cannot be created anew, and only a limited number of people can occupy and use a space at any one time. Copyrighted works are neither scarce nor rivalrous: books are created anew, by specific authors, and can be read by five million people as easily as by five dozen, depriving none of them, nor the author, of the ability to use the work." Critics state that copyright expiration does not deprive a creator's heirs of the right to continue to appreciate