What is Acquiescence? A Theoretical Quest Grounded in Judicial Practice

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Abstract This article examines the concept of acquiescence in international law, as defined by the International Court of Justice as “equivalent to tacit recognition, manifested through unilateral conduct which the other party may interpret as consent.” It aims to contribute to the ongoing debate by analysing the merits of the multiplicity of theories concerning the legal characterisation of acquiescence, investigating whether it can be classified as a source of law, a legal act, a legal fact or a presumption of consent, and distinguishing it from related doctrines such as estoppel and extinctive prescription. The article argues that the main difficulty in addressing these questions lies in the uncertainties surrounding acquiescence as a ‘conceptual unit.’ It contends that, depending on the mereological approach adopted, acquiescence can either be equated by synecdoche with one of its ‘constituent parts,’ or, alternatively, it may be viewed as an autonomous legal concept that transcends the sum of its parts, encapsulating the inherently complex ‘action-reaction’ paradigm that underlies its functioning. This article is divided into three parts. The first part, after some introductory considerations on the illocutionary force of states’ silence, reconstructs the requirements of acquiescence on the basis of international case law. By elucidating the judicial criteria under which states’ silence may be interpreted as acquiescence, this initial ‘empirical analysis’ lays the groundwork for the second part, where the legal characterisation of acquiescence is investigated. Firstly, it will be discussed whether acquiescence can qualify as a legal act or as a legal fact. This will prove useful in distinguishing acquiescence from norms such as estoppel and extinctive prescription. Secondly, the article will examine whether acquiescence can be characterised as a source of law. Finally, in the third part, some concluding remarks will be made on how, depending on the mereological approach adopted, acquiescence may be defined by synecdoche with one of its constituent elements or, alternatively, as a conceptually autonomous sui generis legal phenomenon.