The term used in the title has been coined by analogy with folie à troix.
“Instead of disputing the facts, we must try to explain them. But whatever explanation we offer, we soon find ourselves in very deep waters indeed.”
—H. H. Price19
It has long been known that “true,” or “ordinary,” hallucinations are very common not only in certain psychoses but also in various toxic conditions, infections, etc,; yet only recently has it become clear that some parahallucinatory phenomena, in particular those connected with sleep (e. g., hypnagogic and hypnopompic imagery) also occur oftener than it was generally realized. Many contemporary authors (Ardis and McKellar,2 McKellar,15 McKellar and Simpson,16 and others) express this view. For instance, Smythies22,23 writes: “Visual hallucinations in the form of hypnagogic and eidetic imagery, occur . . . frequently in normal people.” Similarly, Dawson5 states: “Hallucinations occur in