Facts

Semantically the facts constitute a declaration of a true state of affairs. As far as Prolog is concerned, any fact in its database is treated as true. If a file containing the fact

male(phil).

is consulted, the goal

|?-male(phil)

elicits the Prolog response

yes

This is Prolog reporting that the expression evaluates as true with respect to its database. With respect to the same database, the goals

|?-female(phil).

|?-male(chas).

will produce the responses:

|?-female(phil).

no

|?-male(chas).

no

That is, with respect to this database, these facts are not known to be true. A database consisting only of facts is not very interesting. The action picks up when rules are added.