What is the fair use doctrine in the context of copyright law?

not an infringement of copyright. Id. Nevertheless, it is an affirmative defense that assumes copyright infringement has occurred and places the burden on the infringer to show why his use was fair. Monge v. Maya Magazines, Inc., 688 F.3d 1164, 1170 (9th Cir. 2012). In determining whether use of a copyrighted work is protected under the fair use doctrine, courts consider: "(1) the purpose and character of the use, including whether the use is commercial 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." 17 U.S.C. 107.