In some applications it is useful to make the assumption that the database contains all the relevant facts. Therefore, if something cannot be proved to be true, it can be assumed to be false.
Is there an employee of the company whose age is greater than 80?It is probably a good assumption that if no such employee is represented in the database, there is none.
The opposite assumption, the Open World Assumption, assumes that if something cannot be proved we do not know its truth status. This is the normal assumption for logic.