An agent A believes to information I when A holds that I is a good interpretant for some situation S that A is interpreting. This is a primitive of 'processes' because not all interpretants of a S processed by A are believed by A.
(<=> (Believes ?A ?B) (And (Exists (?Z) (And (Occurrent ?Z) (Interpretant-Of ?B ?Z))) (Processes ?A ?B)))