" [T]he intense cathexis of longing which is concentrated on the missed or lost object (a cathexis which steadily mounts up because it cannot be appeased) creates the same economic conditions as are created by the cathexis of pain which is concentrated on the injured part of the body. [...] The transition from physical pain to mental pain corresponds to a change from narcissistic cathexis to object cathexis. An object-presentation which is highly cathected by instinctual need plays the same role as a part of the body which is cathected by an increase of stimulus. The continuous nature of the cathectic process and the impossibility of inhibiting it produce the same state of mental helplessness. "
― Sigmund Freud , Inhibitions, Symptoms and Anxiety