In March 1919, Freud wrote a letter indicating he was working on a new draft marking a major shift in his psychological theories. The psychoanalyst believed he was working on something that would push his theories toward a deeper and more meaningful level. This short essay by Sigmund Freud took more than a year to write. Before this work, Freud’s theories focused on the pleasure principle’s role in driving human behavior. He hypothesized that humans were driven by an instinctual desire to seek out pleasure and avoid pain. His 1899 The Interpretation of Dreams explores this theory. Freud saw a connection between the pleasure principle and libido: He defined libido as an instinctual energy that was ruled by the pleasure principle.

Freud enveloped these ideas in his construction of the psyche as being made of the id, ego, and superego. The libido is housed in the id, the unconscious part of the psyche that is driven by this energy. Freud equated sexual drive to hunger and the will to power; all are part of an instinct for pleasure.