Holding a conference on the logic of international law was a unique opportunity as most work in legal logic is generalist or takes domestic law as the standard case, while international law as a field has some distinctive features. Lacking a formal and centralised structure, international law is the result of the direct action of its addressees: States, international organisations, non-governmental entities, transnational companies, and even individuals. Furthermore, while in domestic law the rules are organised hierarchically, in international law such vertical order is still scarce. Finally, international law undergoes continuous expansion — the proliferation of special regimes can lead to complex challenges to international cooperation, obscuring which applicable rules are determinative in case of a conflict.

In this manner, insights about logic from domestic law may not always be automatically applied to international law. In some cases, a “translation” is necessary. In other cases, the distinctive features of international law demand the development of specific logical tools to deal with its particular matters. In that connection, it is relevant to point out that there is no single logic in the world but many workable logics. Defining which logic is the best for each case depends on the purpose for which that logic is to be used, among other reasons. As such, international law may provide fruitful ground for legal logicians to develop, expand, and test their theories. At the same time, some of the perspectives presented by legal logic may offer valuable tools to international legal scholars for analysing and conceptualising international law.

The speakers focused on diverse topics. On the first day, Lorand Bartels began by speaking about the rules determining the applicability of international law, and Ulf Linderfalk discussed rational reconstruction in international legal reasoning. Next, Valentin Jeutner and Federica Paddeu spoke about the paradoxes of customary international law. Then, Jure Vidmar discussed the (possible) distinction between procedural and substantive rules of international law. Gabriel Lentner then adopted a logocratic account to analyse what makes a “virtuous” argument in international law. David Lefkowitz finalised our first day by discussing the roles of experience and logic in a culture of legal formalism.

On the second day, Antonia Waltermann began by examining reasons for treating international law as a system, and Torben Spaak considered the question of consistency in international law. Then, Jaap Hage analysed the logic of human rights, and Henrique Marcos gave a logical account of the interaction between humanitarian and human rights law. Craig Eggett had the last word in our conference — he zoomed in on the logic of principal and accessory responsibility for international crimes.

Despite the diversity of topics, some common themes were evident throughout the conference. From these themes, we believe we can draw some conclusions.  

(1) There is a logic to international law but not a single logic of international law. 

While international law follows logical rules that allow us to give a rational account of international legal theory and practice, there is not a single, exclusive logic of international law. Instead, there are many possible logics to international law. This account of more than one logic may surprise those who believe that logic is equivalent to deductive syllogisms. But the truth is that a logic is a set of rules and techniques that allow us to distinguish good (correct) reasoning from bad (incorrect) reasoning. There are many kinds of reasoning, and there are many kinds of logic. Most importantly, none of them is intrinsically better than the others. So, there is no a priori definite logical approach to international law.

(2) Logic plays a vital role in international law, but it does not tell the whole story.

Logic studies reasoning and, thus, it can help us distinguish good reasoning from bad. In this respect, logic is prescriptive — it tells us how we ought to reason. Consequently, logic is vital to all people who value good reasoning; this is particularly true for (international) law practitioners and scholars (in short, “lawyers”). The lifeblood of (international) law is argumentation. The work of lawyers is to do their best to structure and advance their claims as well as to find the flaws in the arguments opposing their own. In this way, logic can be a powerful ally in helping lawyers evaluate the qualities of legal arguments. But while valuable, logic alone does not solve legal problems. Logic cannot tell us the correct answer to a legal question. Determining the correct argument in an international legal dispute goes beyond the logical qualities of such arguments and depends on substantive engagement with international law. Consequently, lawyers should perceive logic as a helpful tool, but it is still lawyers with their practical and theoretical knowledge of international law who are best suited to engage and answer legal questions.

(3) Reflection and collaboration between logic and legal expertise is required to determine the standards for good reasoning in international law.

While the knowledge of international lawyers is vital, the work of logicians is critical. This is particularly clear as we realise that there is no pre-defined logic of international law but instead different options of how to conceive of good legal reasoning and the elements that play a role in international legal argumentation. In this respect, careful reflection on the logical standards used in the legal field is paramount. This reflection should, in our view, marry logical and legal expertise and thus requires input from experts in both areas.

With this in mind, we believe that the discussions held during the “Logic of International Law Conference” were already fruitful. Nonetheless, we also know that much more discussion is needed. In this regard, we are currently organising a discussion group with meetings every 3-4 months to continue debating the logic of international law. Our idea is for a presenter to circulate a paper in advance to allow for our meeting to be pre-read and, thus, allow participants to focus on discussing the presenter’s ideas. Please let us know if that would be of interest.

Finally, we want to thank all conference participants for their contributions — not only their presentations but also the debates and deep engagement with each other’s work throughout the two days of our conference.