The principle of functionality reflects the preeminence given in natural systems to its general function over any distinct service or product which is developed to fulfill a function. Frederic Vester provides the example of mitochondria, whose function in human cells is to regulate energy conversion. To fulfill this function, mitochondria sometimes process carbohydrates into carbon dioxide; in other instances they produce amino acids in order to accomplish the same function.
In our commodity-oriented economies, functional units in municipalities often become over-invested in a specific product, and lose sight of effective, alternative ways to fulfill their function. For example, energy utilities often become over-invested in the production of electricity as a way to provide for energy needs, and often resist more efficient ways to fulfill their function, such as the provision of conservation services which reduce demand.