On its macro-*level, realist theory relies on the concept of the *balance of power. This presupposes a common denominator for power in which all its aspects can be coherently aggregated. On its micro-level, realist theory relies on the idea that states are interested in *relative gains of power. For both statements to work, power needs to be measurable. Indeed, such theories require a concept of power akin to the concept of money in economic theory. As Mearsheimer (2001: 12) put it: ‘Power is the currency of *great-power politics, and states compete for it among themselves. What money is to economics, power is to international relations.’ In this analogy, the striving for utility maximisation expressed and measured in terms of money, parallels the *national interest (i.e. *security) expressed in terms of (relative) power.

This central assumption has been challenged by early realist critiques and more recent institutionalist approaches. In an early argument which also anticipates and implicitly criticises the economic analogy in *neorealist theory, Raymond Aron opposed this aggregated concept of power and the underlying power-money analogy. To aggregate power resources, one needs one common scale on which they can be expressed in terms of each other, just as through money. The basis of Aron’s claim is that the different degree of fungibility of money and power resources makes this impossible. The term fungibility refers to the idea of a moveable good that can be freely substituted by another of the same class. Fungible goods are those universally applicable or convertible in contrast to those who retain value only in a specific context. Whereas fungibility seems a plausible assumption in monetarised economies, in international relations, even apparently ultimate power resources like weapons of mass destruction might not necessarily be of great help for getting another state to change its monetary policies.

Aron recognized that economic theory can be used to model behaviour on the basis of a variety of also conflicting preferences. But for him, with the advent of money as a general standard of value within which these competing preferences can be put on the same scale, compared, and traded-off, economists were able to reduce the variety of preferences to one utility function. In world politics, for reasons of its lacking real-world fungibility, power cannot play a corresponding role as standard of value. Not being a standard of value means that power cannot be the currency of great power politics: in the variety of different (power) denominations, actors are never sure about its real value. At best, power is segmented, usable within defined issue-areas. The positioning of power in a general balance is guess-work, certainly no objective indicator from which to deduce rational behaviour.

In response, realists insisted that diplomats had repeatedly been able to find a measure of power and hence the difference is just one of degree, not of kind. Yet even if actors can agree on some approximations for doing exchanges or for establishing power ranks, this is a social convention which by definition can be challenged and exists only to the extent that it is agreed upon. Power resources do not come with a standardised price tag.

With the link from resources to outcomes foregone, the realist link for understanding the international structure goes missing. For a single international power structure relies either on the assumption of a single dominant issue area or on a high fungibility of power resources. Since both are of little avail, it ‘is time to recognize that the notion of a single overall international power structure unrelated to any particular issue area is based on a concept of power that is virtually meaningless’ (Baldwin 1989: 167).

Stefano Guzzini, Power