This paper will examine the role played by public knowledge in the shari‘a courts of Cairo in the late seventeenth and early eighteenth centuries. What I call public knowledge differed from the knowledge of a specific event by an individual person, which often appears in the court records as testimony. Public knowledge was information known by all the people of a neighbourhood – the ahl al-mahalla or sometimes simply the muslimin – or at least a group of the more prominent among them. Public knowledge might relate to an individual person, but it did not usually refer to a specific event. Rather, public knowledge consisted of general information about someone’s character, reputation or status, or about a community’s customs and standards. I use the phrase “public knowledge” in order to capture the ambiguity between verifiable information and widely-held opinion. A statement of this kind of knowledge did not meet the requirements of the shari‘a for testimony, and when it appears in the court records it is usually designated not as testimony (shahada) but simply as a report (khabar). Although it did not strictly speaking constitute “proof,” as it did not conform to the legal concept of testimony, public knowledge could have an impact in a court case in Ottoman Cairo. It could be a used as a weapon by the ahl al-mahalla, in order to enact sanctions against a neighbour who violated their standards of behaviour or morality. It was also used as a resource by judges who needed to confirm identity, the nature of a relationship, or the reputation of litigant. Lastly, it could be drawn on by a litigant to add credibility to his or her claim. In this paper I will explore what kinds of information were presented as public knowledge and what kinds of people presented and used it. I will argue that neighbourhood residents, or perhaps their prominent representatives, sought to use the shari‘a courts to impose and enforce standards of behaviour distinct from, though informed by, the shari‘a. The cases I will discuss suggest that the admissibility of public knowledge in the courts allowed the neighbourhood community to become an important, if unofficial, legal person.