Proving customary norms: ontology and evidence in international law

• Pages: 89-121
• Date: 01 July 2026
• Published date: 01 July 2026
• Author: Piero Mattei-Gentili
• Subject Matter: Derecho Procesal

Quaestio facti. Revista Internacional sobre Razonamiento Probatorio / International Journal on Evidential Legal Reasoning

Año 2026 11 pp. 89-121 DOI: 10.33115/udg_bib/qf.i11.23272

Quaestio facti. Revista Internacional sobre Razonamiento Probatorio

Quaestio facti. International Journal on Evidential Legal Reasoning

Sección: Ensayos

2026 l 11 pp. 89-121

Madrid, 2026

DOI: 10.33115/udg_bib/qf.i11.23272

Marcial Pons Ediciones Jurídicas y Sociales

© Piero Mattei-Gentili

ISSN: 2604-6202

Recibido 22/4/26 | Aceptado 22/05/26 | Publicado online: 04/06/2026

Editado bajo licencia Reconocimiento 4.0 Internacional de Creative Commons

PROVING CUSTOMARY NORMS:

ONTOLOGY AND EVIDENCE IN INTERNATIONAL LAW

Piero Mattei-Gentili*

Instituto de Investigaciones Jurídicas,

Universidad Nacional Autónoma de México.

pieromg@unam.mx

https://orcid.org/0000-0002-5289-1664

ABSTRACT: is article addresses the epistemological and conceptual challenges involved in proving

customary norms in international law. It argues that confusion surrounding usus and opinio juris

stems from a failure to distinguish between the ontology of customary norms and the determina-

tion of their existence. Drawing on legal epistemology, social norm theory, and analytical legal phi-

losophy, the article conceptualizes customary norms as products of socially constituted structures

grounded in shared normative expectations. Proving opinio juris does not require showing that

States regard their conduct as legally mandatory, but rather that second-order normative expecta-

tions are manifested through patterns of practice. e article defends a dual inferential model —

inductive reasoning to establish usus, complemented by abductive reasoning to infer opinio juris—

and proposes a narrative coherence test as a qualitative supplement to the preponderance standard.

KEYWORDS: customary international law, opinio juris, legal epistemology, social norms, standards

of proof.

SUMARIO: 1. INTRODUCTION.— 2. EPISTEMOLOGICAL AND ONTOLOGICAL FOUN-

DATIONS.— 3. THE STRUCTURE OF CUSTOMARY NORMS: 3.1. e Conventional-

* I want to thank the diligent and sometimes demanding reviewers of this paper. eir feedback

was challenging at times, pushing me to be humble and reevaluate my positions, while also encouraging

me to better defend my ideas where needed. As a result, the nal article is substantially improved.

90 PIERO MATTEI-GENTILI

Quaestio facti. Revista Internacional sobre Razonamiento Probatorio / International Journal on Evidential Legal Reasoning

Año 2026 11 pp. 89-121 DOI: 10.33115/udg_bib/qf.i11.23272

ist Framework of Expectations; 3.2. Normative Expectations; 3.3. Normative Attitudes Without

Moral Endorsement.— 4. Customary law. A theoretical subject.— 5. CONCEPTUAL CLARI-

FICATION FOR INTERNATIONAL LAW DOCTRINE: 5.1. e Relevance of the Opinio Ju-

ris; 5.2. e Issue of Inadequate Justications.— 6. THE EVIDENTIARY METHOD AND AP-

PLICATION: 6.1. Inferring Mental States from Repeated Actions; 6.2. Narrative Interpretative

and Approach; 6.3. Towards a Standard of Proof.— 7. CONCLUSION.— BIBLIOGRAPHY

1. INTRODUCTION

Custom, as a source of law, is an intriguing subject for legal theory and doctrine

because it forces us to reconsider how we approach legal issues by posing a distinctive

epistemic problem. In contrast to statutory sources, the main dierence is that, in

the face of a controversy, one must rst prove that a customary legal norm governs

the case; a norm that does not originate in an identiable authoritative act of posit-

ing. at is, while with statutory law the epistemic task of proving is conned to the

quaestio facti (the determination of the factual matters in controversy), customary

law extends the evidentiary inquiry to the quaestio juris (the concerns involved in

determining the applicable law).

e epistemic burden of establishing the quaestio juris underscores the practical

necessity for legal theory to help determine whether these norms are part of the law.

Unlike statutes, customary norms do not originate from an authoritative act of en-

actment. For legal positivism, which denes law in terms of social facts, this raises

the question: what social facts constitute customary norms, and how can legal insti-

tutions identify them? Legal doctrine has developed the «two-element formula» as

an answer: for there to be a customary legal norm, one requires both general practice

(usus) and acceptance as law (opinio juris). is provides guidance but leaves unre-

solved issues, particularly regarding adjudication. First, there is the ontological ques-

tion about the nature of customary norms: How are we understand the elements of

the formula? e structure of proof depends on the structure of what is to be proven.

Once we have an answer, we can consider which epistemological method is adequate

for proving the existence of customary norms and what makes them part of the law.

e fact that custom is a signicant source of international law makes it a com-

pelling case for study, since identifying customary legal norms remains consistently

dicult and highlights the conceptual and epistemological challenges that legal the-

ory should address. e latter reects in the fact that the International Court of

Justice and other tribunals apply the two-element formula inconsistently, sometimes

emphasizing State practice, sometimes opinio juris, without a clear notion of how

these elements interact or what evidence suces to establish either. Scholarship in

international law has addressed these diculties primarily through internal doctrinal

renements, such as distinguishing «traditional» from «modern» custom, proposing

«sliding-scale» approaches in which strong evidence of one element compensates for

weak evidence of the other, and debating whether opinio juris requires belief that

PROVING CUSTOMARY NORMS: ONTOLOGY AND EVIDENCE IN INTERNATIONAL LAW 91

Quaestio facti. Revista Internacional sobre Razonamiento Probatorio / International Journal on Evidential Legal Reasoning

Año 2026 11 pp. 89-121 DOI: 10.33115/udg_bib/qf.i11.23272

conduct is legally obligatory or merely normatively expected. However, these debates

are conceptually obscure and could benet from incorporating insights from legal

theory and conceptual analysis 1.

is article contends that the confusion in methodology regarding customary

international law stems from insucient attention to the connection between on-

tology and epistemology. Scholars and courts conate questions about what con-

stitutes a customary norm with questions about how to prove its existence. is

reasoning fuses the evidentiary outcome with a conclusion about the ontological

requirements for the existence of norms. Courts’ methodological choices — what

evidence they emphasize — do not necessarily reveal what norms are. e result is

conceptual inconsistency: courts apply the «two-element formula» erratically, and,

consequently, the doctrine oers erratic ontological accounts that paper over rather

than resolve the underlying confusion. Drawing on work in social ontology, norm

theory, and legal epistemology, I develop a framework that reconceptualizes the re-

lationship between usus and opinio juris and proposes narrative reconstruction as the

appropriate method for proving customary norms. Customary norms are conceptual

objects: they exist in virtue of participants’ normative expectations. is ontological

clarication has epistemological implications. Proving customary norms cannot be

a matter of simple empirical observation or inductive generalization from instances

of State practice. Rather, it also requires abductive inference. at is, reasoning from

recurrent observable phenomena to the best explanation available for the peculiar

manifestation of certain reactions (normative expectations) within that structure of

concurrent State practices 2.

e article presents three contributions. First, it provides a conceptually rigorous

account of the relationship between usus and opinio juris. I argue that these are not

variables requiring separate proof but rather parts of a single inferential procedure:

practice serves as evidence from which normative expectations can be inferred. is

can be regarded as a solution for the so-called «chronological paradox» (how can

1 Situated within international legal doctrine, D’Amato observes that although some international

law scholars address legal theory, their engagement frequently lacks sucient depth. D’Amato, 1971.

2 2 Charles S. Peirce explained that abductive reasoning involves a form of pragmatic reasoning

that infers causes from eects. In its more contemporary form, it is also called inference to the best

explanation, and, as Lipton proposes, it can be seen as a form of reasoning in which the phenomenon

to be explained provides essential input for believing that a certain explanation is correct (Lipton,

2000). In our case, the very form of States’ conduct supports the idea that normative expectations best

explain the phenomenon. It includes normative cues, like justifying actions and reactions to deviations

from routines. Hence, it is valid to wonder why States justify how they act, why they react to others’

divergent actions? We seek the underlying cause as the best explanation possible. Observing these

reactions reects reciprocal expectations about appropriate actions and about what must be done; we

can infer that a normative framework — specically, a customary norm among these States — is at

play. It is also worth noting, as Jones does, that abductive inference is commonly employed, particularly

in the social sciences, to interpret the existence of social beliefs (such as norms) and to assume certain

rationality among agents (Jones, 2011, pp. 1701-1702).