If several people see the same thing, the assumption is that it has an objective reality. Since every mind is independent, a collective hallucination is virtually a contradiction in terms. The closest thing to it would be group hypnosis: as in one of those performances in which members of the audience are hypnotised together and told to visualise a given scenario. Just the same, I dare say that it would work only at the overall level - that if you interviewed the subjects after the event, you would find that they saw what the hypnotist told them to see in general, but that the details would vary according to each individual's imagination.
Nevertheless, it appears there are times when an entire group can get themselves "psyched up" to have the same visual hallucination, provided it is simple, and this should be factored into any investigation of the alleged paranormal. We shall look at a few examples, starting with the most dramatic.
