We revisit the electrodynamics of resonant high-$Q$ interactions in atomic systems with a view to gaining insights into the design of meta-atoms and hence bulk metamaterials with profoundly different electromagnetic responses. The relevance of phase coherence and nonlinearity in charged systems is emphasized, as is the need to take care over defining how one specifies effective boundaries and cavities that ultimately determine light–matter interactions. Radically new material properties become apparent once one designs organized clusters of small numbers of atoms or meta-atoms for which the usually applied random phase approximation (RPA) does not apply. The RPA relies on averages in sufficiently large volumes consisting of large numbers of interacting systems, while our model assumes a small volume with averages in time, i.e., ergodicity. New meaning is given to the concept of effective and practically useful constitutive parameters, based on this very fundamental point of view, which is important to metamaterials.