"The Unconscious Mind"
Sigmund Freud revolutionized our understanding of the self. He compared the mind to an iceberg: the conscious mind is just the tip above water, while the vast Unconscious lies beneath, driving our desires and behaviors.
He divided the psyche into three parts: The Id (primal instincts, pleasure principle), the Superego (morality, conscience), and the Ego (reality, mediator). Mental struggles arise from the conflict between the Id's desires and the Superego's restrictions. Though many of his specific theories are outdated, the concept of the unconscious remains central to psychology.