Rationality is often said to be a normative concept: to be rational is not a descriptive feature of an individual or a system, but a normative one, since rationality pertains to what one ought to, or should, do, think or feel. But what is it in turn for an individual or system to ought to do, think, or feel something? I propose to start from this problem and try to see in what sense the notion of normativity can illuminate those of rationality and reason. Here I shall limit myself to the normativity which is attached to the content of thoughts or mental contents, not to actions. But first, I need to say something about normativity in general . Although it is notoriously problematic, the distinction between descriptive expressions/concepts and normative expressions/concepts is familiar. “Cat”, “car”, “tree”, are descriptive, but “nice”, “stupid”, “bad” are normative. Very often sentences or kinds of propositions are said to be descriptive or normative. Declarative sentences, used to make assertions, are descriptive, but imperative sentences and those which contain normative terms are said to be normative. Within the domain of normative concepts it is common to distinguish two classes: a) evaluative or axiological concepts, such as “good”, “bad”, “desirable”, among which philosophers also distinguish “thin” concepts (“good”, “bad”) and thick concepts (“courageous”, “coward”); b) deontic concepts, such as “right”, “ought”, “permissible”, “forbidden” (deontic concepts are by definition thin). All these expressions are either used as predicates or as operators on sentences. A further question is: to what do normative concepts apply? Primarily, they apply to actions, to what to do, what to want, what to desire, what to avoid. In which case normativity is mainly a feature of the practical domain. But it seems also to apply to the theoretical domain, to what to think, what to believe, what to know, what to judge. It also applies to feelings: what to feel, what to experience, what not to suffer, what to enjoy. There is a standard of taste as well: what to praise, what to like and dislike, etc.