One of the most interesting developments in patent law during the last century is the catastrophic collapse of the paper patent doctrine, which had authorized courts to discriminate against patents that were never successfully practiced by their patentees. The doctrine's demise opened the door for the dramatic and controversial rise in patent litigation by "nonpracticing entities" or "patent trolls"-entities that, in the words of President Obama, "don't actually produce anything themselves." This Article undertakes a comprehensive review of this lost doctrine and shows that the doctrine took a balanced approach, hurting patentees who never developed their technologies but helping those who had. The doctrine declined because it could not be reconciled with the theoretically impoverished views about information disclosure embraced by courts of the late twentieth century. With the advent of more sophisticated theories about the value of learning-by-doing and, more generally, about the problems associated with generating and disseminating information, the paper patent doctrine now has what it lacked in the past-a solid theory for favoring patents that were taught not just through paper disclosure but also by real-world practice. This Article concludes that the paper patent doctrine should be revived and that existing case law provides a sufficient foundation for a revival.