Masson--that the statement applies to all nine books of the letters

Momsen--reversed this theory:

that the nine books are in chronological order, that each book belongs to a specific period and that the letters in it are in order --that the statement in I.1 is a literary gesture intended to give the air of artificial carelessness to the first volume.

Arguments--

1. some letters [within books and between consecutive books] form a series of chronologically arranged letters about particular topics

2. in each book there are some letters that can be exactly dated by context

Fresh evidence from inscriptions and papyri and the advance of prosopographical study which has clarified the careers of many officials mentioned in the letters have tended to confirm Mommsen in general, but not to confirm his strict chronology.

Guillemin--perhaps Pliny is disguising the care with which he had arranged the sequence of letters in book I for effect and variety

Pliny may mean that unlike the editors of Cicero's letters [which he claims as his model] he has made variety rather than chronology the guiding principle of the internal arrangement