This paper is a report of a study designed to explore the way in which community and hospital based nurses categorised 35 medical conditions. The results, obtained from a series of card-sorts using a Multiple Sorting Task and a modified Q-Sort, found that nurses used four global sub-categories of severity to categorise them. The findings provided a basis for an analysis of nursing assessment and established that they were realised as prognostic judgements. Discussion of the structure of the categories led to a possibility that the cognitive skill referred to as 'intuition' could be explained by the way in which clinical knowledge comes to be organised in memory.