abstract. The constitutionality of a search or seizure typically depends upon the connection between the target of that search or seizure and some allegation of illegal behavior—a connection assessed by asking whether the search or seizure is supported by probable cause. But as central as probable cause is to the Fourth Amendment’s administration, no one seems to know what it means or how it operates. Indeed, the Supreme Court insists it is “not possible” to define the term, holding instead that the probable-cause inquiry entails no more than the application of “common sense” to “the totality of the circumstances.” Viewed charitably, this refusal to elaborate on the meaning of probable cause stems from an understandable desire for doctrinal flexibility in the face of weighty and competing law-enforcement demands. But the Court’s doctrinal approach is also routinely criticized as an “I know it when I see it” jurisprudence that is ill equipped to safeguard civil liberties in the numerous interactions between civilians and law-enforcement actors.

This tension between doctrinal flexibility and structure is the animating dilemma of probable cause jurisprudence—a dilemma that this Article attempts to navigate and, ultimately, to resolve. To do so, it urges a rejection of an often invoked—if not always followed—tenet of Supreme Court doctrine: probable cause unitarianism. That dominant idea, expressly endorsed in many of the Court’s leading precedents, holds that whatever probable cause means, it ought to entail the same basic analytic method and be judged by the same substantive standard, from one case to another. But on close inspection, the Supreme Court’s existing jurisprudence contains seeds of an alternative— and superior—conception of probable cause, which this Article terms probable cause pluralism. On this view, probable cause is an open-textured and capacious idea that can comfortably encompass distinct analytic frameworks and substantive standards, each of which can be tailored to the unique epistemological and normative challenges posed by different types of Fourth Amendment events. Probable-cause analysis can be statistically driven or intuitively assessed; it can demand compelling evidence of illegal behavior or only an occasionally satisfied profile; it can presume the credibility of some types of witnesses while treating others with deserved skepticism or disbelief. It can, in short, come to mean something—if it gives up on meaning any one thing in all cases.

In its current form, probable cause’s pluralism is nascent, implicit, and undertheorized—and is thus at best a stunted and haphazard collection of disparate ideas. This Article’s central contribution is to bring those ideas together, refining and synthesizing them into a comprehensive account of what a pluralist theory of probable cause could and should look like. Specifically, by organizing probable cause around three central analytic axes—which in turn ask how to assess evidentiary claims, how to assess proponents of such claims, and how to determine the certainty thresholds for those two assessments—this Article constructs a universally applicable framework for determining the constitutionality of any given search or seizure. With that framework in hand, scholars and jurists will be better equipped to reason through the many and varied cases to come and better able to assess the many cases that have come before.

author. Professor of Law, Harvard Law School. I am grateful to many colleagues whose thoughtful comments helped to improve and refine this Article: Paulina Arnold, Monica Bell, Rabia Belt, Nikolas Bowie, Kevin Cole, Andrew Ferguson, John Goldberg, Andrew Hellman, Elizabeth Kamali, Orin Kerr, Genevieve Lakier, Cynthia Lee, Benjamin Levin, Anna Lvovsky, John Manning, Justin Murray, Catherine Padhi, John Rappaport, Richard Re, Daphna Renan, Daniel Richman, Adam Samaha, Abigail Shafroth, Jocelyn Simonson, Adrienne Spiegel, Carol Steiker, Jordan Woods, Crystal Yang, and David Zionts. My thanks as well to the participants in the St. John’s University Law School Faculty Colloquium and the Harvard Law School Criminal Justice Workshop for helpful comments on prior drafts. I am also grateful to my diligent research assistants who helped me compile and analyze the cases in this Article’s appendix: Gavan Duffy Gideon, Riley Majeune-Fagan, Grace McLaughlin, Steven Palmer, and Parisa Sadeghi. This Article went to print as the U.S. Supreme Court was considering a potentially related case, Kansas v. Glover, 139 S. Ct. 1445 (2019). For an application of some of the ideas expressed here to that case, see Brief for Professor Andrew Manuel Crespo as Amicus Curiae in Support of Affirmance, Kansas v. Glover, 139 S. Ct. 1445 (2019) (No. 18-556), 2019 WL 4256217, reprinted in Andrew Manuel Crespo, The Unavoidably Empirical Fourth Amendment: A Case Study of Kansas v. Glover, 1 Cts. & Just. L.J. 217 (2019).