The paper refers to the discussion on the influence of the postmodernism onto psychotherapy. We are looking at the notion of truth from the point of view of mathematical logic and we argue with the postmodern concept of truth. From the logical point of view truth is a very delicate notion. Dealing with it carelessly leads to a contradiction. It turns out that a safe use of the notion of truth requires a clear distinction between our opinion on the reality from the reality itself. The truth is the relation of compatibility between a description and the reality. In psychology and psychotherapy the notion of truth understood as the adequacy of our perception of ourselves and of other people is crucial. If the human psyche has its own internal logic, then, possibly, the mathematical laws concerning truth apply to it and similar difficulties occur. In mathematical logic the deepest trouble occurs when self-reference is considered. On the other hand, this is the core of psychotherapy: the subject studies himself. It turns out that a certain intra-psychical splitting of roles into the role of a studying subject and the role of a studied object is needed. Psychoanalysis invented here the notion of an observing ego. However, we are approaching here the limit of the possible use of the notion of truth. This can be seen as the difficulty of applying the notion of truth to the transference-feelings: are they true or not?