"...every psychological theory, apart from what it achieves from the point of view of natural science, must fulfil yet another major requirement. It should explain to us what we are aware of, in the most puzzling fashion, through our 'consciousness' and, since this consciousness knows nothing of what we have so far been assuming - quantities and neurones - it should explain this lack of knowledge to us as well."

Freud, S,1897. 'The Project for a Scientific Psychology. Standard Edition, 1: section [7] 'The Problem of Quality'