The selection principle and the only prerequisite for establishing the interaction system is Presence based on Perception. Since no absent person can contribute to interactive communication, the foundational distinction for observing interaction is the distinction between those present and those absent, although not everyone who is present must necessarily take part in communication (the bar tender does not necessarily participate in communication between customers at the bar). The distinction present/absent allows the formulation of a relatively simple definition of the Boundary of Communication: **Interaction is the simplest Social System**. Interaction is, however, still a complex social system because the number of possible communications makes a Selection necessary [→Complexity].
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Interaction is a Social System that requires the physical presence of the participants. Interactions form when communication is based on the perception of presence, which ensues from →Double Contingency.