of operation on a well-known psychological premise. This premise is
that to get a reaction from one of the senses there must be a
stimulus. If you think you see a UFO you must have seen something.
Pure hallucinations are extremely rare.

For anything flying in the air the stimulus could be anything that
is normally seen in the air. Balloons, airplanes, and astronomical
bodies are the commoner stimuli. Birds and insects are common also,
but usually are seen at such close range that they are nearly always
recognized. Infrequently observed things, such as sundogs, mirages,
huge fireballs, and a host of other unusual flying objects, are also
known stimuli.

On Project Blue Book our problem was to identify these stimuli. We
had methods for checking the location, at any time, of every balloon
launched anywhere in the United States. To a certain degree the same
was true for airplanes. The UFO observer's estimate of where the
object was located in the sky helped us to identify astronomical
bodies. Huge files of UFO characteristics, along with up-to-the-
minute weather data, and advice from specialists, permitted us to
identify such things as sun-dogs, paper caught in updrafts, huge
meteors, etc.

This determination of the stimuli that triggered UFO sightings,
while not an insurmountable task, was a long, tedious process. The
identification of known objects was routine, and caused no
excitement. The excitement and serious interest occurred when we
received UFO reports in which the observer was reliable and the
stimuli could not be identified. These were the reports that
challenged the project and caused me to spend hours briefing top U.S.