A library that is not arranged becomes disarranged…
In practice, every library is ordered starting from a combination of these modes of classification, whose relative weighting, resistance to change, obsolescence, and persistence give every library a unique personality.
We should first of all distinguish stable classifications from provisional ones. Stable classifications are those which, in principle, you continue to respect; provisional classifications are those supposed to last only a few days, the time it takes for a book to discover, or rediscover, its definitive place. This may be a book recently acquired and not yet read, or else a book recently read that you do not quite know where to place and which you have promised yourself you will put away on the occasion of a forthcoming “great arranging,” or else a book whose reading has been interrupted and that you do not want to classify before taking it up again and finishing it, or else a book you have used constantly over a given period, or else a book you have taken down to look up a piece of information or a reference and which you have not yet put back in its place, or else a book that you cannot put back in its rightful place because it does not belong to you and you have promised several times to give it back, etc.