In many cases, rules can be interpreted as logical statements: if X is true then Y is true. We can then think of the rules as a programming language based on logic. The inference engine runs this program. That's the idea behind Prolog (``PROgramming in LOGic'') and related languages. Often linked to the idea that you tell the computer what to do, rather than as in (e.g.) Basic or Pascal, how to do it.