All complex systems are, by definition, made up of a number of interacting parts. In general, these components vary in their type, structure, and function within the whole system. Thus a system’s behavior cannot be summarized simply by adding up the behavior of the individual parts. Contrast a simple physical system (say, an ideal gas) with a complex biological system (say, an organism). The temperature of the gas is a simple aggregation of the kinetic energy of all the individual molecules in the gas. The temperature, pressure, and volume of the gas are related by