The purpose of copyright law, according to the U.S. Constitution, is to "promote the Progress of Science and useful Arts," by granting authors the exclusive right to control their works "for limited Times." Over the years, with successive amendments of the Copyright Act, that"limited time" has grown longer and longer, and the scope of exclusive control has increased. But the Copyright Act has also evolved certain exceptions that permit courts to avoid applying it rigidly when doing so would stifle the creativity it is designed to foster. The best known of these is the "fair use" doctrine, which first arose at common law and is now codified in §107 of the Copyright Act. But although the fair use doctrine is certainly well known, it is not always well understood—not even by the courts that have to interpret it.