Because energy is dissipated when anything happens, it is a substantive process that provides a common denominator for all activities, human and nonhuman. The second law of thermodynamics defines the irreversibility (and therefore in an important sense, directionality) of the model and thus is a measure for everything that happens. Energy can be thought of as a material cause of everything, but in an instrumental, or agential, sense energy causes nothing; it merely requires that all events evolve so as to reduce the capacity to do work that may be available in the system. This provides a measure for the analysis of human history, one that is culture free and value free and uncolored by political or moral concerns. This permits comparison among all historically known societies of any culture.
Although not always explicit, two perspectives— the agential and holistic—have been used to describe the relationship between energy and human society. The first treats energy as a resource that is controlled by human beings to secure their survival and adaptation, i. e., to sustain their life. Seen in this way, the environment is full of energy resources— biota (including human beings), fossil fuels, water and solar radiation, subnuclear sources, etc.—that human beings harness and control to work for them. Somewhat problematic in this picture is human labor, because individuals control their own labor and may control the labor of others. Thus, when referring to the energy per capita expended by a society in a contemporary industrial nation, we are usually talking about nonhuman expenditure, but when dealing with preindustrial societies and particularly in hunting and gathering societies, human labor is the major energy source. Two phases emerge from this situation: (1) energy (including one’s own energy) as controlled by individual human beings and (2) energy as controlled by society, usually by some agent for the collectivity. In complex societies (i. e., with two or more levels of social integration), leaders or authorities decide how energy resources, both nonhuman and human collectivities, are to be used. This perspective readily separates human energy expenditure as an agential process that controls and is impacted by on-going energy-based events.
In the holistic perspective, human beings and the society and the environment relevant to them are a continuing and fluctuating field of multiple energy flows. Within this field, there are islands or sectors of potential energy (materials, fuels, stored goods) held in equilibrium, the energy of which may be released for human use when an appropriate technology is applied. There are also dissipative structures (see later) wherein human labor is seen as merely one part of the flow, as simply one more energy resource. In this perspective, the triggers (see later) that release potential energy are energy flows. This perspective allows us to see social processes in terms of a series of principles, laws, and working hypotheses derived from the physical and biological sciences. If sociocultural evolution is merely one facet of general biological evolution, then these concepts and principles should be equally applicable to social evolution.