The most nonsensical notion, the most casual slip of the tongue, the most fantastic dream, must have a meaning and can be used to unriddle the often incomprehensible maneuvers we call thinking. In 1974, he published another book.

A glance at its chapter headings will indicate some of the aspects of behaviour covered by the book:Forgetting of proper names, Forgetting of foreign words, Childhood and concealing memories, Mistakes in speech, Mistakes in reading and writing.

Broadly, Freud demonstrates that there are good reasons for many of the slips and errors that we make. We forget a name because, unconsciously, we do not wish to remember that name. We repress a childhood memory, because that memory is painful to us. A slip of the tongue or of the pen betrays a wish or a thought of which we are ashamed.

Freud was intent not merely on originating a sweeping theory of mental functioning and malfunctioning, he also wanted to develop the rules of psychoanalytic therapy. As to the first, he created the largely silent listener who encouraged the analysand to say whatever came to mind, no matter how foolish, repetitive or outrageous, and who intervened occasionally to interpret what the patient was struggling to say.

The efficacy of analysis remains a matter of controversy, though the possibility of mixing psychoanalysis and drug therapy is gaining support.