The oldest part of the brain, the core brain stem, responsible for instincts and drives, roughly corresponds to Freud's id. Freud claimed the psyche was structured into three parts (the id, ego and superego) that develop at different stages in our lives.
Freud's id consists of the biological components of personality and is the unconscious impulsive part of the psyche that responds to instincts.
The id contains a human's basic, instinctual drive, which is not affected by reality, logic or the everyday world. The id operates on the "pleasure principle", which is the idea that every impulse needs immediate satisfaction, regardless of the consequences.
Sigmund Freud explored the human mind more thoroughly than any other who became before him. His contributions to psychology are vast.
He was one of the most influential people of the twentieth century and his enduring legacy has influenced not only psychology, but art, literature and even the way people bring up their children.
Learn more about Sigmund Freud's theories on SimplyPsychology.org.