It would seem, from a psychological point of view, to be an important circumstance in the genesis of a false perception whether the intellectual process sets out from within or from without.

And it will be found, moreover, that this distinction may be applied to all the varieties of error which I propose to consider.

Thus, for example, it will be seen further on that a false recollection may set out either from the idea of some actual past occurrence or from a present product of the imagination.
It is to be observed, however, that the line of separation between illusion and hallucination, as thus defined, is a very narrow one.

In by far the largest number of hallucinations it is impossible to prove that there is no modicum of external agency co-operating in the production of the effect.

It is presumable, indeed, that many, if not all, hallucinations have such a basis of fact.