The indexing problem changes with each new book undertaken. To meet the needs of different classes of seekers and to suit various types of books, rules entirely satisfactory in one case must be varied in the next and perhaps ignored or even reversed for a third... Indexing is a highly complex intellectual process involving the use of language in a specific and somewhat artificial way, and that it is also to a considerable extent a matter of intuition, the workings of which cannot be reduced to fixed rules. It is 'knowing what but not knowing how'.