Thirdly, if one event precedes another in time, the first is perceived as a cause of the second and the connection between the two becomes a cause and effect relation. After a period of time, impressions may be recalled. There are two forms of this reminiscence or recollecting process. On the one hand, there is memory, a faculty whereby impressions return with a vivacity which is half way between impressions and ideas. When something is remembered, the impression is called up, not quite as vivid, but more vivid than a figment of the imagination.
Impressions always occur, when they do, before an idea.
But an idea, on the other hand, may exist without an impression. For example, one might have the idea of a mermaid or a purple dragon without ever having a corresponding impression or sensation. Psychotic episodes and dreams are other examples of ideas without impressions. Jung's notion of archetypes, however, would seem to suggest that even bizarre ideas might result from some racial past. However, it is an observed fact that expected events have recurred.