The ‘feel’ of an interactive system can be compared to the impressions generated by a piece of music. Both can only be experienced over a period of time. With either, the user must abstract the structure of the system from a sequence of details. Each may have a quality of ‘naturalness’ because successive actions follow a logically self-consistent pattern. A good composer can write a new pattern which will seem, after a few listenings, to be so natural the observer wonders why it was never done before. Read more – ‘Hansen’s User Engineering Principles for Interactive Systems’.