How big is human memory, or on being just useful enough

We are, in many respects, what we remember. But how much do we do? So far, science has provided only a very partial answer to this riddle. The magical number seven, plus or minus two, seems to constrain the capacity of our immediate memory (Miller 1956). But surely its constraints dissipate when memories settle in long-term stores. Yet how big are these stores? If we combine all of our factual knowledge and personal reminiscence, childhood scenes and memories of the past day, intimate experiences and professional expertise-how many items are there, that, combined together, mold us into unique individuals? The answer is not simple, and neither is the question. For example, what is an item in long-term memory? And how can we measure it, being sure that we unveil memory capacity and not merely the occasional ability to tap it? Such theoretical and practical difficulties, no doubt, have contributed to the fact that the capacity of human memory is still an enigma. Yet, despite the inherent and undeniable complexities, the issue deserves to be retrieved, once in a while, from the oblivions of the collective memory of the scientific community. (For a selection of earlier discussions of the size of human long-term memory, see Galton 1879; Landauer 1986; Crovitz et al. 1991.)