This distinction, which has been adopted by the majority of recent alienists[1], is a valuable one, and must not be lost sight of here.  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.  Thus, the madman who projects his internal thoughts outwards in the shape of external voices may, for aught we know, be prompted to do so in part by faint impressions coming from the ear, the result of those slight stimulations to which the organ is always exposed, even in profound silence, and which in his case assume an exaggerated intensity.  And even if it is clearly made out that there are hallucinations in the strict sense, that is to say, false perceptions which are wholly due to internal causes, it must be conceded that illusion shades off into hallucination by steps which it is impossible for science to mark.  In many cases it must be left an open question whether the error is to be classed as an illusion or as a hallucination.[2]

For these reasons, I think it best not to make the distinction between illusion and hallucination the leading principle of my classification.  However important psychologically, it does not lend itself to this purpose.  The distinction must be kept in view and illustrated as far as possible.  Accordingly, while in general following popular usage and employing the term illusion as the generic name, I shall, when convenient, recognize the narrow and technical sense of the term as answering to a species co-ordinate with hallucination.

Departing, then, from what might seem the ideally best order of exposition, I propose, after all, to set out with the simple popular scheme of faculties already referred to.  Even if they are, psychologically considered, identical operations, perception and memory are in general sufficiently marked off by a speciality in the form of the operation.  Thus, while memory is the reproduction of something with a special reference of consciousness to its past existence, perception is the reproduction of something with a special reference to its present existence as a part of the presented object.  In other words, though largely representative when viewed as to its origin, perception is presentative in relation to the object which is supposed to be immediately present to the mind at the moment.[3] Hence the convenience of recognizing the popular classification, and of making it our starting-point in the present case.