ELIZA was a computer program and an early example (by modern standards) of primitive natural language processing. ELIZA operated by processing users' responses to scripts, the most famous of which was DOCTOR, a simulation of a Rogerian psychotherapist. In this mode, ELIZA mostly rephrased the user's statements as questions and posed those to the 'patient.' ELIZA was written by Joseph Weizenbaum between 1964 to 1966.
In DOCTOR mode, ELIZA might respond to "My head hurts" with "Why do you say your head hurts?" The response to "My mother hates me" would be "Who else in your family hates you?" ELIZA was implemented using simple pattern matching techniques, but was taken seriously by several of its users, even after Weizenbaum explained to them how it worked. It was one of the first chatterbots in existence.1