The joke, as Zizek tells it, goes along these lines:
A man is convinced he is a grain of seed. He is quickly taken to a mental institution where the doctor eventually convinces him that he not a grain of seed; he is a man. He is then supposedly cured and is permitted to leave the hospital. However, once he steps outside, he immediately rushes back in trembling with fear. “There is a chicken outside,” the man says “and he is going to eat me.” The doctor tells him, “Come now, you know very well you not a grain of seed, but a man.” “You and I surely know that,” the man tells him, “but does the chicken know?”
This just tells us the nature of psychoanalytic study — it not enough to convince the truth to the patient, but one must also be convinced that others assume that same truth. It is this struggle of truths that encapsulates the psychiatric field, which attempts to normalize individuals who have accepted a reality different than that of one’s peers.