Ronald Syme, "Pliny and the Dacian Wars," in his Roman Papers
, VI (Oxford: Clarendon Press, 1991), pp. 142-149 (at 142):
Certain fields of study, large or small, have an easy appeal because no
fresh information has accrued through the ages, or seems likely to emerge.
Negative evidence can
be variously instructive. It is often worth the effort to look for the words
and phrases an author avoids, the persons, topics, or episodes he has decided
to omit. That is a clue to his style and idiosyncrasy. Also and further,
a revelation of social conventions or political attitudes.