Systemic Integration as Philosophical Holism: Why International Law refuses to live in Separate Apartments

International lawyers often speak about "fragmentation" as though it were a disease. The language itself is revealing. Something has fractured. Something has broken apart. Something once whole has become divided. The diagnosis is fragmentation; the search is for a cure. Yet perhaps the problem is easier to understand through a more familiar analogy.

Imagine a city where every public authority insists on obeying only its own rulebook. The traffic department ignores environmental concerns because it only deals with roads. Environmental regulators ignore property rights because they only deal with trees. Tax authorities disregard constitutional guarantees because someone else worries about those. Each institution performs its own function perfectly, but nobody is responsible for ensuring that the city works as a whole. The result would be chaos.

International law occasionally looks uncomfortably similar. Investment lawyers speak investment law. Trade lawyers speak trade law. Human rights lawyers speak human rights law. Climate lawyers speak climate law. Each regime develops its own institutions, vocabulary and priorities. Left unchecked, international law risks becoming a collection of neighbouring apartments whose occupants spend more time arguing over the corridor than maintaining the building.

Article 31(3)(c) of the Vienna Convention on the Law of Treaties is international law's attempt to stop the neighbours from bricking up their doors. The provision requires treaties to be interpreted in light of "relevant rules of international law applicable in the relations between the parties." Lawyers know this as the principle of systemic integration.

The International Law Commission identified it as one of the principal responses to fragmentation. Most scholarship treats it as a doctrine of treaty interpretation designed to preserve coherence among increasingly specialised legal regimes. That account is undoubtedly correct. But it leaves a deeper question unanswered. Why is coherence considered desirable in the first place?

Why do international lawyers instinctively resist the idea that investment law, environmental law, trade law and human rights law should evolve in complete isolation from one another? The answer, I suggest, lies beneath doctrine. Systemic integration is not merely a rule of treaty interpretation. It is the legal manifestation of a much older philosophical commitment: holism.

At its core, holism rejects the idea that meaning can be understood by examining isolated parts alone. The meaning of a sentence depends upon the language in which it is spoken. The meaning of a word depends upon its place within the sentence. The meaning of a legal rule depends upon the legal system within which it operates.

Yet much legal argument proceeds in precisely the opposite direction. Parties often invite tribunals to treat a treaty provision as though it had washed ashore from a desert island, detached from every other norm of international law. The exercise becomes one of textual archaeology: excavate the clause, identify the words and declare the matter settled.

Systemic integration proceeds from a fundamentally different intuition. It assumes that legal norms derive meaning not merely from their text but from their relationship with other norms operating within the same legal order. That assumption has deep philosophical roots.

The story perhaps begins with Ludwig Wittgenstein [Philosophical Investigations (Oxford: Blackwell, 1953), §§65–71]. Wittgenstein famously challenged the idea that concepts require a single defining characteristic. Instead, he proposed the notion of "family resemblance." Members of a family may share eye colour, mannerisms, expressions or temperament. No single trait is common to everyone, yet the resemblance remains unmistakable.

International law functions in much the same way. Investment law, environmental law, trade law and human rights law do not derive from a single constitutional source. They were negotiated by different actors, for different purposes and often at different times. Yet they repeatedly employ familiar concepts: good faith, proportionality, due process, abuse of rights, legitimate expectations and sustainable development. The regimes are not identical. They are relatives. Systemic integration recognises these family relationships. It does not create them.

If Wittgenstein explains why regimes resemble one another, W.V.O. Quine explains why they cannot remain isolated.

Quine rejected the notion that knowledge exists as a collection of independent propositions. Instead, he described human understanding as a "web of belief" in which individual propositions derive support from their connection to the wider network. Pull one strand and the entire web moves. International law increasingly resembles such a structure [W.V.O. Quine and J.S. Ullian, The Web of Belief, 2nd ed. (New York: Random House, 1978), 8–28].

Trade measures affect environmental regulation. Environmental regulation affects investment protection. Human rights obligations influence migration policies. Climate commitments influence energy regulation. No regime operates in complete isolation because the underlying facts themselves refuse to respect disciplinary boundaries. Lawyers often attempt to divide the world into neat categories. Reality rarely cooperates. Systemic integration acknowledges what Quine would likely have regarded as obvious: legal norms exist within a broader network of meaning. They cannot be understood entirely apart from that network.

Hans-Georg Gadamer takes the argument a step further. His concept of the hermeneutic circle rests on a deceptively simple insight. We understand the whole through its parts, but we also understand the parts through the whole [Truth and Method, 2nd rev. ed. (New York: Continuum, 1989), 268–307]. Neither comes first.

Anyone who has assembled furniture without first reading the instructions will appreciate the problem. The purpose of a particular component only becomes clear when one understands the finished structure. Yet understanding the finished structure requires understanding the role of the individual components. Treaty interpretation often operates in precisely this manner.

A tribunal cannot fully understand a particular treaty provision without understanding the wider legal environment within which it operates. At the same time, the content of that legal environment is informed by the treaties that comprise it. Article 31(3)(c) of the Vienna Convention effectively institutionalises this process. The provision invites interpreters to move continuously between text and system, provision and principle, part and whole. Systemic integration is therefore not merely an interpretive technique. It is a practical application of a broader hermeneutic insight.

The final piece of the puzzle comes from Ronald Dworkin [Law’s Empire (Cambridge, MA: Harvard University Press, 1986), 225–275]. Even if legal systems are interconnected, one might still ask why coherence should be preferred over fragmentation.

Dworkin's theory of law as integrity provides the answer. For Dworkin, legal interpretation should strive toward the reading that best fits and justifies the legal system as a whole. Law derives legitimacy not merely from isolated rules but from the coherence of the principles that underlie them.

Viewed through this lens, systemic integration becomes more than a tool for managing normative conflict. It becomes an expression of legal integrity. The doctrine reflects an intuition that international law should aspire to function as a legal system rather than as a collection of disconnected bargains. Coherence is not pursued because fragmentation is aesthetically displeasing. It is pursued because coherence promotes principled adjudication. Dworkin supplies the normative justification for what Article 31(3)(c) requires in practice.

This philosophical perspective casts much of the existing literature in a different light. The leading doctrinal accounts of systemic integration often appear highly technical. Yet beneath the technical language lie unmistakably holistic assumptions.

Campbell McLachlan's influential account of Article 31(3)(c) assumes that legal meaning emerges from a wider normative environment ['The Principle of Systemic Integration and Article 31(3)(c) of the Vienna Convention,' International and Comparative Law Quarterly 54 (2005): 279–320].

Martti Koskenniemi's fragmentation study presupposes that international law remains a legal system despite increasing specialisation [Difficulties arising from the diversification and expansion of International Law, Report of the Study Group of the International Law Commission finalised by Mr. Martti Koskenniemi Document A/CN.4/L.682 and Add.1].

Joost Pauwelyn's work on normative conflict proceeds on the basis that competing norms must ultimately be situated within a common legal framework [Conflict of Norms in Public International Law (Cambridge: Cambridge University Press, 2003)].

Stephan Schill's multilateralization thesis similarly treats investment treaties as components of a broader legal order rather than isolated bilateral bargains [The Multilateralization of International Investment Law (Cambridge: Cambridge University Press, 2009)].

The philosophers and the international lawyers are engaged in remarkably similar conversations. They simply speak different languages.

The significance of this debate becomes particularly apparent in climate-related investment disputes. Many investment treaties were drafted long before contemporary climate governance emerged. They pre-date the Paris Agreement. They pre-date net-zero commitments. They pre-date the widespread acceptance of climate mitigation as a central objective of public policy.

Yet tribunals increasingly confront disputes arising directly from climate-related regulation. Renewable energy reforms. Fossil fuel phase-outs. Carbon reduction measures. Environmental restrictions on resource extraction.

The recent wave of climate-change themed investor state arbitrations illustrates this dynamic with particular clarity. In cases such as Rockhopper v. Italy and RWE v. Netherlands, tribunals have been required to confront regulatory measures adopted in pursuit of climate objectives while simultaneously applying investment protections negotiated in a very different political era. These disputes reveal the futility of treating investment law and climate governance as separate apartments. The environmental protection measures adopted by the state was the investors cause of action. A purely atomistic approach risks viewing environmental regulation only as a potential interference with investor rights. A holistic approach recognises that both investment protection and climate mitigation form part of the broader normative landscape within which state conduct must be assessed.

The same tension is visible in the renewable energy arbitrations arising from regulatory reforms in Spain, including Eiser v. Spain, Antin v. Spain and Novenergia v. Spain. Although often framed as disputes concerning legitimate expectations and regulatory stability, they also expose a deeper jurisprudential question. Can treaty standards be interpreted without reference to the evolving commitments that increasingly shape national energy policy? Systemic integration suggests not.

The significance of these cases lies not  merely in their outcomes, but in their demonstration that contemporary investment disputes are increasingly embedded within a wider legal ecosystem in which climate obligations, sustainable development and economic regulation interact continually rather than independently.

The question is no longer merely doctrinal. It is conceptual. Should investment treaties be interpreted as legal time capsules, frozen at the moment of their creation? Or should they be interpreted against the backdrop of the broader international obligations that now shape state conduct?

Systemic integration offers a principled answer. It functions less like a formal amendment and more like a software update. The text remains the same. The interpretive environment evolves.

Dworkin explains why climate obligations cannot simply be ignored. Gadamer explains how they become part of the interpretive process. Quine explains why investment law cannot be isolated from the wider normative network. Wittgenstein demonstrates that both regimes already share a common vocabulary of good faith, proportionality and sustainable development. Systemic integration therefore provides the mechanism through which investment law adapts to climate governance without rewriting treaty texts.

None of this suggests that fragmentation is always undesirable. Specialised regimes exist for good reasons. Environmental law pursues objectives that differ from those of investment law. Human rights law focuses on concerns that may not always align with trade liberalisation.

A legal system does not become coherent by pretending that disagreements do not exist. Nor does systemic integration authorise tribunals to rewrite treaties in pursuit of abstract harmony.

The doctrine is better understood as establishing a presumption of coherence rather than a mandate of uniformity. The goal is not to eliminate differences. It is to ensure that differences remain intelligible within a common legal framework.

Lawyers often describe systemic integration as a conflict-management tool. That description is accurate but incomplete. The doctrine performs a more fundamental task. It reminds international law that it is a legal system rather than a collection of enthusiastic specialists shouting across disciplinary fences. Wittgenstein explains why legal regimes resemble one another. Quine explains why they cannot remain isolated. Gadamer explains how interpreters move between text and system. Dworkin explains why coherence remains a worthwhile aspiration. Viewed in this way, Article 31(3)(c) is more than a rule of treaty interpretation. It is the legal expression of philosophical holism. Or, to return to the original analogy, it is international law's standing reminder that however much the neighbours disagree, they still live in the same building.

Mikhail Behl is a Counsel, Arbitrator & Mediator operating out of the Chambers of Mikhail Behl.