In this paper two questions are raised : 1) Under which conditions may we say that an entity is a representation of something else ? and, in particular, 2) what is the guarantee that our (mental or internal) representations correspond to or are in accord with external objects ? In Hume's opinion, three answers to the second question are possible : There is a causal relationship between represented objects and representations ; representations are freely created by the mind ; and God guarantees the connection between representation and represented things and events. A short historical examination of these ideas is proposed. In cognitive sciences the two questions are strictly related. In particular, Dretske, Lloyd, and Perner's positions are...