This paper discusses the view that fantasizing, understood as a flight into fantasy, belongs to a type of mental functioning distinct from imaginative fantasy. From this the idea emerges, proposed by Winnicott, that withdrawal into fantasy assumes a dissociative quality, which is formed early on as a defensive solution following the loss of hope in object relations. Such a defence becomes the foundation for a dangerous enclave in which the individual ends up enclosing himself, experiencing an illusory self-sufficiency. In this perspective, the author maintains that the flight into fantasy must be understood as a risk factor for the draining of the self or for a crystallization into psychopathological structures, becoming an automatic activity of 'non-thought' that substitutes for the working-through processes necessary for the development of the mind. The paper investigates this psychopathological dynamic, which was already present in Breuer and Freud's writings, examining subsequent contributions of various authors. Clinical material (of both children and adults) illustrates how the flight into fantasy may take the form of an anti-relational realm of the mind, compromising the operations necessary to the integration of psychic life. There is also a discussion of which therapeutic tools may help the patient to gradually abandon the withdrawal in favour of an authentically nourishing relational nature.