Traced back to Aristotle (384-322 BP), it is a method of data collection in which observers record their own mental processes and experiences. Refined by Wilhelm Wundt (1832-1920) from uncontrolled to controlled introspection (or rather immediate retrospection), it was finally banished from psychology by the behaviourists and John B. Watson (1878-1958) in particular.
See Behaviorism, Psychology