Turkuler Isiksel’s wide-ranging, erudite, and measured account of the political and legal evolution of the European Union invites us to probe the limits of constitutional theory and practice. The book argues that rather than producing a common form of constitutionalism based on individual rights and popular sovereignty, the EU has produced a distinctive form of constitutional practice, one that Isiksel labels “functional constitutionalism.” To the extent that the EU has constitutional features, those are the result of its functional imperative, namely, that of creating an economic union, which consists of a single market, monetary union, financial coordination, and shared consumer protection practices.

The EU’s constitutional features fall under three pillars. First, EU treaties enjoy legal supremacy, which means that they take precedence over the laws of member states in case of conflict. Second, the EU engages in supranational judicial review through the European Court of Justice (ECJ) and national courts, which invalidate national laws that do not conform to EU treaties. Third, EU law has direct effect, which means that it creates rights that can be claimed by individuals before national courts. Indeed, the extensive jurisprudence and practice of allowing individuals to access and exercise their rights of economic mobility and their associated social benefits has elevated the citizens of member states into “the custodians of obligations member states owe to one another” (p. 61).

Isiksel makes a strong case that the EU legal order is best understood through a functional logic, which means that its institutions are valued not as vehicles of popular will or universal rights, but primarily as mechanisms for “expeditious decision-making, competent administration, and efficient provision of public goods” (p. 87). According to this logic, constitutions do not merely constrain the exercise of political power but articulate and enable it. By describing EU constitutionalism in functionalist terms, Isiksel seeks to expose a central, if often tacit, basis of constitutional authority.

The possibility of articulating this functional logic stems from the author’s insistence that constitutionalism is a dynamic category, and that we should not be bound in our understanding of the concept by the limited and limiting practice of domestic constitutional orders. But she is well aware of the trade-offs with the other common expressions of constitutionalism and of the dangers of stretching constitutionalism too much. In fact, one can stretch this functional logic at the expense of individual rights and popular sovereignty, to allow a legal order to devolve into authoritarian rule, which always and everywhere excuses the expansion of political power as an instrument of effective government.

I will raise a few questions about whether the functional logic outlined by Isiksel is sufficient to characterize a legal order as constitutional. But for now, let us note that Isiksel persuasively documents the role of the ECJ as the leading institutional mechanism that has converted the EU legal order into a constitutional one, insofar as this label is apposite. Ever since the Van Geld en Loos decision launched a regime of individual rights that gave EU treaties direct effect into national legal orders, the ECJ has generated a regime of supranational rights of mobility on the basis of states’ reciprocal commitments to enhance the freedom of movement for goods, capital, services, and persons across member states’ borders. Isiksel shows with a sharp eye and painstaking research into the history of the ECJ that market freedoms have come to enjoy the status of fundamental norms of the EU legal order, due to their centrality for achieving the objective of an economic union.

The market freedoms protected by the EU are distinctive from, and at times have come into conflict with, the human rights that form the basis of domestic constitutional orders. In fact, member states have drawn on their constitutional protections for individuals to resist ECJ encroachment on their sovereign prerogatives. This, in turn, has prompted the ECJ to arrogate unto itself human rights jurisdiction beyond market freedoms, in order to reduce the ability of national courts to resist its authority on human rights grounds. It has claimed that the protection of fundamental human rights is an “existential requirement for the EU legal order” (p. 112). And herein lies the first problem with the claim that the EU is a constitutional order: The ECJ claims exclusive authority to adjudicate the bounds of its supranational legal authority. This is, if anything, an anticonstitutional feature of the EU, because it allows the institutions that exercise power to define the bounds of that power. The ECJ is paradigmatic for its extraconstitutional practice of expanding the role of EU treaties into the national law of member states, and in the process profoundly altering member states’ domestic institutions and constitutional values and limiting their ability to set policy priorities that conflict with their supranational legal obligations.

Although it seems in theory that EU treaties limit the ECJ’s ability to expand the prerogatives of the supranational institutions indefinitely, by limiting them to the purpose of building an economic union, in practice the ECJ and other EU institutions have insisted that almost every policy area is in fact implicated in creating a single market, and thus falls within EU jurisdiction. EU supranational institutions have taken advantage of the ambiguities of the treaties to further limit member state competencies by wading into areas “only tenuously related to EU’s areas of competence,” as Isiksel readily acknowledges (p. 111). In fact, the ECJ has constitutionalism precisely backwards. It has engaged in repeated extralegal, extraconstitutional expansions of its power, and its “spontaneous ‘arrogation’ of a human rights mandate” (p. 108) bears this out.

The more fundamental question is whether functional constitutionalism can stand on its own as a distinctive form of constitutionalism. It could potentially do so if the enabling feature was unique to constitutional rule, but it is not. Arguably, it is more a feature of regular legislation. Regular legislation routinely authorizes the exercise of political power to ensure the safety of the citizens, to resolve conflicts, to encourage economic development, and to collect and distribute public money, among other tasks. Thus, regular legislation, and by extension its equivalent at the international level (namely, state-based treaties), can define and empower the exercise of political power without thereby transforming a legal order into a constitutional one. Therefore, the existence of treaties negotiated by member states to functionally enable the creation and expansion of the economic union are neither necessary nor sufficient to transform the EU into a constitutional order. If we thought that regular legislation that empowers the exercise of political power could do that, we would be falling into the trap of christening any legal order with the label “constitutional” simply by virtue of its functionally differentiated institutional machinery, and this would be stretching the concept too far. This means that the EU could at most be characterized as a quasi- or proto-constitutional order.

Despite my doubts about functional constitutionalism, I have benefited enormously from this carefully researched and powerfully argued book. Europe’s Functional Constitution is reassuringly balanced: neither infected by wild-eyed integrationist fervor, for which no cost to achieve “an ever-closer union” is too large, nor marred by the cynicism of EU critics who see it as merely an attack on state national interest. For all these reasons, the book is an indispensable guide for thinking about the possibility of law and constitutional practice beyond the borders of sovereign states.