Between Lex Lata and Lex Ferenda? Customary International (Criminal) Law and the Principle of Legality

Tilburg Law Review: Journal of International and European Law, Vol. 15, No. 2, pp. 163-183, 2011

25 Pages Posted: 6 Dec 2011 Last revised: 8 Dec 2011

See all articles by Noora Arajärvi

Noora Arajärvi

University of Oxford - Centre for Socio-Legal Studies

Date Written: February 5, 2011


International courts serve an important function in finding and interpreting existing law, but as noted already forty years ago the focal problem of international courts is that: “the borderlines between interpretation of existing law and the making of new law are inevitably fluid.” Particularly in interpreting customary international law the courts face situations which call for innovative methods in determining the definition and the scope of the law. International criminal law has only recently grown more coherently codified, but still bears much on customary law elements, unlike some other fields of public international law, in which rules have been quite thoroughly codified. Additionally, in criminal law issues such as foreseeability, and non-retroactivity, to mention a few, ought to be given a high credence, or the reason that it is the liberty of an individual that is at stake. In discovering and applying customary international law, international criminal courts are confronted with the balancing exercise between following positivist approach – attaching itself to law as it is, as enacted or adopted by an authority, and may lead to an undesired, even unjust outcome in criminal trials – and natural law approach – arising from inherent rights and duties regardless of their formal recognition, and may risk the violation of the principle of legality. Principle of legality determines the limits of the law-making process and encompasses norms that legal rules must adhere to, such as clarity, promulgation and non-retroactivity.

The question arises, how can the courts preserve the principle of legality, and especially the prohibition of retroactive effect of law, in interpreting – and in formulating – customary international law in a progressive manner. Should the principle of legality be applied liberally in order to bring perpetrators to justice, or is strict application necessary in criminal courts, even at the cost of some impunity? This paper analyses how these two aspects should be balanced and reconciled against one another in discovering, interpreting, and applying customary international law, with reflections on theories of H.L.A. Hart and Lon Fuller – the two influential proponents of modern positivist and natural legal thought, respectively – and recent case-law of international courts in fields of international criminal law and human rights law. Also, Hart‟s primary and secondary rules, and rule of recognition are considered in the determination of what may count as customary international law. First, I consider the principle of legality and its importance in the formation of law – with emphasis on customary international law, followed analysis of its link to morality and the concepts of lex lata and lex ferenda. I have included a brief discussion on the application of primary and secondary rules and rule of recognition in the formation of (customary) international law. Finally, I analyze selected recent court decisions which deal with international crimes and the principle of legality, with the aim of reflecting on the above issues.

Suggested Citation

Arajärvi, Noora, Between Lex Lata and Lex Ferenda? Customary International (Criminal) Law and the Principle of Legality (February 5, 2011). Tilburg Law Review: Journal of International and European Law, Vol. 15, No. 2, pp. 163-183, 2011, Available at SSRN: or

Noora Arajärvi (Contact Author)

University of Oxford - Centre for Socio-Legal Studies ( email )

Wolfson College
Linton Road
Oxford OX2 6UD, OX1 3UQ
United Kingdom

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