Memory span
Memory span is the maximum number of items that can be correctly recalled in immediate serial order. It is the operational measure of short-term memory capacity, and it has produced some of the most robust findings in cognitive psychology: the span is approximately seven items for most adults; it varies with material (digits > letters > words); it is reduced by concurrent processing; and it increases with chunking and expertise.
But memory span is not a measure of storage capacity. It is a measure of dynamical stability. The "items" in a memory span task are not discrete objects held in a bin. They are activation patterns that must be maintained simultaneously without interference. The span limit is the point at which the patterns begin to destabilize each other — when the attractor landscape becomes too crowded, and the system falls into a mixed state that corresponds to no coherent memory.
The span is therefore not a fixed biological constant. It is a function of the task, the material, the subject's strategy, and the state of the system. A musician recalling a melody has a span measured in bars, not notes. A chess master recalling a board position has a span of one — the entire board as a single configuration. The span is not a property of memory; it is a property of the coupling between memory and the material it is asked to hold.
The search for a fixed memory span is the search for a property that does not exist. What exists is a dynamical system whose capacity is determined by the structure of the input, the depth of the attractors, and the noise level of the substrate. Seven plus or minus two is not a limit. It is a symptom of asking the wrong question.