He theorized that objective loss, such as the loss of a valued relationship through death or a romantic break-up, results in subjective loss as well; the depressed individual has identified with the object of affection through an unconscious, narcissistic process called the "libidinal cathexis" of the ego.
Sigmund Freud likened the state of melancholia to mourning in his 1917 paper "Mourning and Melancholia".