Robert Ayson is Director of Centre for Strategic Studies at Victoria University, Wellington. 

The recent Interpreter debate over the relative power of China and the US is important in its ambition but questionable in some of its method. The problem is the assumption that we can decide whether the US really is in relative decline on the basis of an economic comparison with a growing China, and on then finding some way of determining how that material power makes for influence.

Even if we were to agree on the same metric for measuring that economic power, we would still have a variable where each unit of power is equal. But power is about relationships (what I can do to you?; what do you think I can do to you?; what do others think I can do to you and to them?). It is not about comparisons (am I bigger than you?; do you think I am bigger than you?; do others think I am bigger than you?). We are doing too much of the latter sort of analysis.

Because it is about social relationships, power is as much about perception as it is about concrete capacity: I don't perceive you as powerful simply on the basis of your size or other material factors (compare Japan and Israel for a moment). And because it is about perception, power is lumpy rather than smooth (as Thomas Schelling might say). Some units of power are much more equal than others. Changes in power status do not simply appear gradually and cumulatively. Large symbols of change, which shock and otherwise alter our perceptions, are often more important than a collection of small concrete ones.

Let us take as an example Britain's decline in Asia. Unlike today's debate over what China's rise means for America's influence, this example of decline is incontestable.

Britain's economic exhaustion after the second of two world wars in the first half of the century was part of a relative economic decline that was both systemic and perceptible. But, at least looking backwards, people are less inclined to remember the 1967 sterling crisis than the physical withdrawal of British forces east of Suez. Like the failure of Britain's Singapore strategy a generation before, this was a huge, lumpy, change which affected others. It was symbolic of that wider systemic weakness, but that weakness was not enough on its own to guarantee it. It took a political decision: an act of will.

So much of the current debate is about the search for signs of those underlying and slow-moving systemic trends (as if we really can detect them properly) rather than the critical dimension of real decline: the political inability or unwillingness to sustain influence.

Of course, this can happen when material power has run out: the sheer will to use power is not enough if there is not going to be anything to wield. But it can also be caused by a reluctance to influence others even when there is material power there to utilise. And this seems to me to be the big question: not how the US and China measure up materially, but how they measure up politically and psychologically.

This leaves open the possibility of a shock even if our economy-watching is as good as it gets. I think its an open secret that both the US and China will have massive and globally important economies in a generation's time and more. But that only gets us so far. And if it is unwise to extrapolate China's recent growth rates it may be even more hazardous to apply a similar linear logic to Mr Obama's pivot in Asia.