Within the Prolog universe predicate may be divided into to primary classifications: facts and rules. A fact is accepted as an axomatic truth within the Prolog universe. Many simple Prolog applications use thse facts for their knowledge base to find solutions. Complex applications may assert and retract facts dynamically to provide simplier reasoning.
Prolog facts are defined by using a predicate head followed by a peroid.
The omission of the predicate defintion infix oeprator
:- as well as the boyd implies truth when the variables in the head may be bound to the query in question.
Besides the short hand syntax, there is nothing particularlly special about facts from the logical perspective. Facts follow the standard rules for Prolog inference rules for backtracking.