This paper argues that the ordinary notion of belief plays a dual role, Both as a state with semantic (propositional) content, And as an internal psychological (functional) state of individuals, And that most attempts to construe belief as a scientifically respectable notion cannot reconcile propositional content with psychological role. This duality of belief is responsible for many of the indeterminacies of belief ascriptions and is therefore one of the most serious obstacles for the project of artificial intelligence and for scientific psychology.