The purpose of United States copyright law is “[t]o promote the Progress of Science and the useful Arts.” U.S. Const., Art. I, § 8, cl. 8. This purpose is achieved not only through protection of copyrighted works, but also by allowing “some opportunity for fair use of copyrighted materials.” Campbell v. Acuff-Rose Music Inc., 510 U.S. 569, 575 (1994). Accordingly, the current Copyright Act includes a defense that insulates the “fair use of a copyrighted work” for “purposes such as criticism, comment, news reporting, teaching ... scholarship, or research.” 17 U.S.C. § 107. The act identifies four nonexclusive factors that “shall” be considered in evaluating fair use: “(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes; (2) the nature of the copyrighted work; (3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and (4) the effect of the use upon the potential market for or value of the copyrighted work.” Id.