The development of cognitive science bring about many problems. One of them is necessity of ontological differentiation in relation to such types of reality as dreams or a virtual reality are. If we want to characterize a world of dreams from the ontological point of view we have to clarify two questions: What do dreams represent(?); and what is the criterion of truth if we are thinking about the world of dreams? The answers on these questions are in the center of the article. The ontological differentiation enables not only the inner differentiation of cognitive science but also find out the consistent view on human cognition.