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  • Ideas in the Brain: The Localization of Memory Traces in the Eighteenth Century
  • Timo Kaitaro

Plato suggests in the Theaetetus that we imagine a piece of wax in our soul, a gift from the goddess of Memory. We are able to remember things when our perceptions or thoughts imprint a trace upon this piece of wax, in the same manner as a seal is stamped on wax. Plato uses this metaphor to explain the errors which arise when we mistake something for something else: we connect the perception of an object with the trace belonging to another. The metaphor can also be used in explaining differences in people’s mnestic capacities: rapid learning and forgetting correspond to soft wax, impure wax results in muddled traces, etc.1 If we locate the traces in the brain instead of in the soul, Plato’s metaphor gains consistency and turns into a testable hypothesis. This move was already made by Quintilian.2

So, the metaphor of memory as traces in the brain is evidently not a modern invention. This is easy to understand. To frame the hypothesis one needs only to reflect on how we use objects outside our brains for mnemonic purposes. We conserve ideas by tracing or printing letters and words on paper. We are also able to conserve images in drawings, paintings and prints. What could be more natural than to think of memory as the formation of traces in the brain? The development of even better information storage techniques in the form of pictures, symbols, and signs provides the metaphor extra plausibility. Plato’s seal allows us to imagine the imprint of a person’s likeness, but modern techniques of information storage and retrieval make it even easier to imagine memory in general as the formation of material traces.

Explaining memory in terms of material traces could, of course, be taken [End Page 301] to mean that the formation of material traces in the brain is merely a necessary condition of memory. But one could also take a more reductionistic stance which identifies memory with the formation of material traces. Furthermore, one could identify specific mental items (sensations, ideas, memories) with specific physiological events or anatomical entities in the brain. This seems especially plausible if we imagine that the traces in the brain are as discrete and separate as ideas or memories are in our minds. Since the traces we form or print on paper consist of discrete letters and words, the thought that the traces in the brain are equally discrete and separate comes easily. Plato, in fact, made separateness a condition for the memory to work properly: it is not easy to read signs printed on one another.3 Of course, no traces are actually visible in the brain. But they are easy to imagine, as it is to imagine explanations of psychological phenomena, such as association, based on these traces.

At the end of the seventeenth century, explanations of mental phenomena referring to material traces in the brain were used by writers, from Platonists to materialists, who supported widely different theories about the nature of the human mind.4 In fact, as I will show later in this paper, the question of the possible identity of mental entities, like ideas or sensations, with material entities or traces in the brain is relatively independent of the question of the existence of an immaterial soul. Or rather, if there is a systematic connection between the answers proposed to these two questions in the eighteenth century, it is not the connection one would expect. Nowadays we often tend to think of a prototypical materialist as someone who makes radical reductionistic claims about the identity of mental phenomena with brain states. We also tend to think of the eighteenth-century French materialists as “mechanistic.”5 In view of this reputation, it may come as a surprise to find that some eighteenth-century materialists, notably Denis Diderot, were in fact consistent anti-reductionists and opposed to mechanical or mechanistic explanations of the mental.6 In fact, the epithet “mechanistic” is misleading even when applied to the philosopher who is traditionally presented as the paradigmatic example of a “mechanistic materialist,” Julien Offray...

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