Though neither program was developed as a narrative work, the software programs ELIZA (1964–1966) and SHRDLU (1968–1970) can formally be considered early examples of interactive fiction, as both programs used natural language processing to take input from their user and respond in a virtual and conversational manner. ELIZA simulated a psychotherapist that appeared to provide human-like responses to the user's input, while SHRDLU employed an artificial intelligence that could move virtual objects around an environment and respond to questions asked about the environment's shape. The development of effective natural language processing would become an essential part of interactive fiction development.