Memory, its physical structure, is an unsolved challenge. It is, perhaps, the central question… But theories about the nature of memory have still not really progressed beyond the stage of description through analogy. Analogy has often been a valuable step in the discussion of biological problems, but it is, of its nature, constrained by the technological development of the time or the level of scientific knowledge in other fields… Even current models of memory dwell on analogies with existing, artificial methods of storing information. Mental memory has been compared with the electro-magnetic polarisation of ferrite rings in the core memory of a computer, and with the ‘distributed’ image of a hologram – a device that stores a record of a three-dimensional scene by photographing the interference pattern produced by illuminating it with laser light… In any case, most theories of memory, whether couched in terms of mere analogy, or even in terms of the storage of information in networks of real nerve cells, concentrate on the manner in which events can cause changes in physical structures. In other words, they are concerned with the machinery of memory, not the code – the symbolic form in which the events are registered… Most theories of memory are, as it were, concerned with the question of ink and paper and not with the much more fundamental issue of the grammar of remembrance.