Where Freud made everyone aware of the goings-on within their hidden, individual selves, Carl Jung popularized the idea that everyone – most, if not all, of the human race – draws from a common pool of ideas, images, and meanings. This reservoir is known as the Collective Unconscious.
The concept is that there are ideas which mean the same thing (basically) to people scattered about in different nations and even different time periods. The basic form these ideas take are as archetypes, which often take the visual form of a graphic or symbol – i.e., a wheel, a sword, a flame, etc. Such archetypes are supposedly part of our makeup even without our needing to know about or experience them, much like instinct in animals. (For example, baby chicks know the shapes of the shadows of predatory birds as they fly overhead, and start to panic without quite realizing why.)
Naturally, Jung and his ideas have their critics, many of whom argue that his archetypes are rather Eurocentric, and thus perhaps not so universal after all. Some impart mystical or quasi-supernatural origins to the C.U.; others aver that it is merely a consequence of humans being part of a highly organized society.
Jung himself sought to explain the UFO phenomenon in his book A Modern Myth of Things Seen in the Sky – as one might judge from the title, he saw UFOs as being part of the Collective Unconscious, spefically the circular shape of the things making them a manifestation of the mandala, a sacred symbol of universal unity. In any case, Jung saw them not as tangible, physical objects existing in reality, and naturally many UFO believers – particularly those who hold them to be extraterrestrial spacecraft – disagree strongly. But, one could hardly blame him: he was a psychologist, not an investigator.