" With these points in mind, it’s helpful to more closely examine the relations between grammars, theories of physical systems, and generated systems. Grammatical rules determine the meaningful orderings of words within a language, thereby defining the corpus for the language. Similarly, the mechanisms of a physical model (anything from levers to electron spin) determine possible trajectories through physical-state space (such as the trajectory of a probe through the solar system). It is possible to mimic grammatical rules and physical mechanisms in a generated system by specifying appropriate operators for the system. Once the appropriate operators are chosen, we can make precise comparisons between corresponding grammars, physical models, and generated systems. The generated system format offers an additional advantage because it encompasses additional important complex systems, such as computer programming languages. An important advantage of precise comparisons is that activities that are easy to observe in one complex system often suggest ‘where to look’ in other complex systems where the activities are difficult to observe. "

― , Complexity: A Very Short Introduction