What is a database?
This quote is at the beginning of 'Temporal Data and the Relational Model'.
"The database is not the database -the log is the database, and the database is just an optimized access path to the most recent version of the log."
B.M. Schueler
An interesting thing of working with developers is that they always try to find generic answers to questions, so one of them came up with this generalization of the previous one ;):
"The Present is not the Present - the Past is the Present, and the Present is just an optimized, compact representation of the Past."