QUOTE : The essential point is that systems of knowledge are never given in their entirety at the outset of an investigation. Their meaning unfolds only gradually, through the exploration of their implications with the help of external representations - for example, by doing calculations, applying a conceptual framework to new circumstances, exposing systems to debate within an epistemic community, and so forth. We therefore speak of the 'generative ambiguity' of external representations, which leads to their seemingly paradoxical function in knowledge transmission: on the one hand, they secure and stabilize the transmission of knowledge over generations, thus ensuring its longevity, while they serve, on the other hand, as tools for thinking and thus agents of change and innovation in the transmission of knowledge.