In Kleinian theory unconscious phantasies underlie every mental process and accompany all mental activity. They are the mental representation of those somatic events in the body that comprise the instincts, and are physical sensations interpreted as relationships with objects that cause those sensations. Phantasy is the mental expression of both libidinal and aggressive impulses and also of defence mechanisms against those impulses. Much of the therapeutic activity of psychoanalysis can be described as an attempt to convert unconscious phantasy into conscious thought.
Freud introduced the concept of unconscious phantasy and phantasising, which he thought of as a phylogenetically inherited capacity of the human mind. Klein adopted his idea of unconscious phantasy but broadened it considerably because her work with children gave her extensive experience of the wide-ranging content of children’s phantasies. She and her successors have emphasised that phantasies interact reciprocally with experience to form the developing intellectual and emotional characteristics of the individual; phantasies are considered to be a basic capacity underlying and shaping thought, dream, symptoms and patterns of defence.
Reproduced from The New Dictionary of Kleinian Thought by Bott Spillius, E., Milton, J., Garvey, P., Couve, C. and Steiner, D. (Routledge, 2011)