The usefulness of arithmetic stems directly from two facts. Firstly, numbers are inherent in many different phenomena. Secondly, arithmetic is a familiar and well-developed body of theory. So, in seeking units of complexity, we need to look for constructs that are widely applicable, familiar, and have the capacity for development of useful methodologies. There are several structures that meet these requirements.
Strings provide a conceptually simple respresentation for many processes.
Examples include temporal patterns, such as animal behaviour or system state changes, natural language and DNA sequences. As work on L-systems shows, many growth processes can be displayed as sequences. More generally, string grammars have been used to model a wide variety of processes. Graphs are perhaps the most fundamental units. Note that both strings and arrays have an implicit graph structure.