The ego provides the immediate reaction to the events of reality. The ego is the first line of defense between the self and the outside world. It tries to balance the two extremes of the id and the superego.

BOB DOUGHTY: Many of Freud's theories about how the mind works also had strong sexual connections. These ideas included what he saw as the repressed feelings of sons toward their mothers and daughters toward their fathers.

If nothing else, Freud's ideas were revolutionary. Some people rejected them. Others came to accept them. But no one disputes his great influence on the science of mental health.

Professor James Gray at American University in Washington, D.C. says three of Freud's major ideas are still part of modern thinking about the mind.

One is the idea of the unconscious mind. Another is that we do not necessarily know what drives us to do the things we do. And the third is that we are formed more than we think in the first five years, but not necessarily the way Freud thought.

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FAITH LAPIDUS: Doctor Freud was trained as a neurologist. He treated disorders of the nervous system. But physical sickness can hide deeper problems. His studies on the causes and treatment of mental disorders helped form many ideas in psychiatry. Psychiatry is the area of medicine that treats mental and emotional conditions.

Freud would come to be called the father of psychoanalysis.