Introduction

The 1969 Vienna Convention on the Law of Treaties represents a cornerstone of international law, codifying interpretive principles in Articles 31 to 33, which emphasize the treaty’s text, context, object, purpose, and good faith. However, international courts frequently rely on supplementary interpretive tools not explicitly mentioned in the Convention, such as a contrario interpretation. The maxim of a contrario interpretation, encapsulated in expressio unius est exclusio alterius (the express mention of one thing excludes others), serves as a pivotal yet understudied tool in the interpretation of international treaties. Rooted in logic and common sense, this maxim posits that the deliberate inclusion of a specific provision or term in a legal text implies the exclusion of unmentioned provisions or terms. Its application, while intuitive, is fraught with complexities, as overuse or misapplication can lead to interpretations that diverge from the drafters’ intent or the treaty’s broader objectives.

Research Gap and Objective

In Persian legal scholarship, research on this topic is notably scarce, with only a handful of studies, such as Piri and Azizi (2021), or the correspondent author of this research’s MA thesis (2021), addressing it. This study taking inspirations and guidance mainly from the work of Joseph Klingler et al namely Between the Lines of the Vienna Convention? Canons and Other Principles of Interpretation in Public International Law addresses these gaps by offering a comprehensive typology that captures the multifaceted nature of a contrario interpretation and evaluates its persuasive force across diverse judicial contexts.

Methodology

The research adopts a descriptive-analytical approach, combining doctrinal analysis of legal texts with a case-law-based examination of international jurisprudence. Primary sources include judgments and advisory opinions from the ICJ, PCIJ, ECtHR, Iran-United States Claims Tribunal, and other arbitral bodies, such as those under the Permanent Court of Arbitration (PCA) and the International Centre for Settlement of Investment Disputes (ICSID). Secondary sources encompass scholarly articles, treatises on treaty interpretation, and legal commentaries. The typological framework is derived inductively from the analysis of judicial reasoning, identifying patterns in how courts apply a contrario interpretation. Each typology is assessed for its logical coherence, contextual appropriateness, and alignment with the principles of the Vienna Convention.

Key Findings:

The study identifies four distinct typologies of a contrario interpretation, each reflecting a unique application of the maxim in international jurisprudence:

1. Absence of an Item in a List Indicates Its Exclusion: This typology posits that the omission of a specific item from an enumerated list in a treaty implies its intentional exclusion. It is the most widely accepted form of a contrario reasoning, frequently applied due to its logical clarity. For instance, in the PCIJ’s Appeal from a Judgment of the Hungaro-Czechoslovak Mixed Arbitral Tribunal (1933), the Court held that the explicit mention of “jurisdiction and merits” in Article 10 of the Paris Agreement No. 2 excluded procedural matters from the Court’s purview. Similarly, the ICJ’s advisory opinion in Conditions of Admission of a State to Membership in the United Nations (1948) interpreted Article 4 of the UN Charter as establishing exhaustive conditions for membership, excluding additional criteria not listed. However, this typology is not universally persuasive, as commercial arbitration cases often reject such reasoning when lists are illustrative rather than exhaustive.

2. Explicit Mention of an Item Excludes Opposing or Different Matters: This form of a contrario interpretation suggests that the express inclusion of a provision excludes its opposite or unrelated matters. It is the most common typology, rooted in the classical expressio unius maxim. In Wimbledon (PCIJ, 1923), the Court ruled that only the explicitly listed restrictions in Article 381 of the Treaty of Versailles could limit navigation, excluding unmentioned obstacles. Conversely, in LaGrand (ICJ, 2001), President Guillaume cautioned against overgeneralizing this typology, emphasizing its limited applicability to specific contexts. This typology’s frequent use underscores its intuitive appeal, though courts occasionally reject it when it risks overly restrictive interpretations.

3. Mention in One Part of a Treaty and Omission in Others Suggests Non-Applicability: This typology involves comparing different provisions within the same treaty, inferring that the mention of a term in one section and its absence in another indicates intentional exclusion from the latter. In the Frontier Dispute (Burkina Faso/Niger) (ICJ, 2013), Burkina Faso’s argument that a treaty’s silence on a specific boundary demarcation implied a straight-line connection was rejected, highlighting the need for contextual analysis. Similarly, in the PCIJ’s Interpretation of the Statute of the Memel Territory (1932), the Court avoided a contrario reasoning when comparing Articles 10 and 17, prioritizing textual coherence over exclusionary inferences. This typology requires careful consideration of the treaty’s structure and purpose to avoid misinterpretation.

4. Mention in One Treaty and Absence in Another Indicates Non-Applicability: The least conventional typology, this form infers exclusion by comparing provisions across different treaties. In Right of Passage over Indian Territory (ICJ, 1960), the Court noted that the absence of phrases like “permanent sovereignty” in Article 17 of a 1779 treaty, unlike other treaties, indicated no intent to transfer sovereignty. Similarly, in Grimm v. Iran (Iran-US Claims Tribunal, 1983), the Tribunal concluded that the omission of personal injury claims in a treaty, compared to other treaties explicitly including them, excluded such claims from its jurisdiction. This typology is less persuasive due to the variability in treaty drafting contexts, requiring robust evidence of intentional omission.

Contribution to the Field

By offering a comprehensive four-fold typology, this article fills a void in Persian literature, where prior works (Piri and Azizi 2001) focused on general application rather than precise classification. It advances existing knowledge through comparative judicial analysis, introduces novel perspectives on limitations (e.g., conflicts with good faith), and equips judges and interpreters with a practical toolkit. Departing from formalist approaches, it stresses integration with preparatory works and legal outcomes, attention to the context and object and purpose of the treaty, thus enriching treaty interpretation scholarship.

Conclusion

The maxim of a contrario interpretation, while not codified in the Vienna Convention, is a critical tool in international jurisprudence. Through its four typologies, this study illuminates the maxim’s diverse applications, strengths, and limitations. By advocating for cautious and context-sensitive use, the research equips legal interpreters with a robust framework for navigating the complexities of treaty interpretation. As international law continues to evolve, this typology will serve as a valuable resource for ensuring interpretations that are both legally sound and aligned with the normative goals of global governance.