 It is quite clear that, in a strong sense of “know,” the travel agent does not know that there are no such flights. What is at work here is a tacit assumption that the database is complete, and that since the database does not list any direct flights between the two cities, there are none. A useful way to look at this process is as a kind of minimization, i.e., an attempt to minimize the extension of a given predicate (“flight-between,” in this case). Moreover, on pain of inconsistencies, such a minimization needs to take place not with respect to what the database explicitly contains but with respect to what it implies. 