[Xiyin Tang] Abstract: The fair use defense is frequently invoked as the most important First Amendment safeguard in copyright law, and the effect a defendant’s use has upon the potential market for the copyrighted work is often cited as the most important factor in that determination. Yet courts and commentators alike have struggled with how to define the potential market for a copyrighted work, either underdefining, overdefining, or altogether failing to define what that market is, or should be.