There are two kinds of measurement: ontic--meaning finding out how big or small a thing is using a scale, beginning point and unit; and ontological--more of an experience than an act, when we sense that things don't "measure up" to what they could be. The argument is that anything complex cannot be described through ontic measures, though we may think we have perfected them. We are tempted to seek all meaning through ontic measuring, perhaps because we have not made similar gains in developing ontological measurement methods.
This has implications for how we approach measuring progress toward sustainability, especially as we describe it as relationships in a complex system. Are scales and indices appropriate to describe something like "community capacity"? Can it be measured empirically? Is such an exercise worthwhile? Or should we pursue an ontological method?