In the present volume an attempt will be made to work out the psychological side of the subject; that is to say, illusions will be viewed in their relation to the process of just and accurate perception.  In the carrying out of this plan our principal attention will be given to the manifestations of the illusory impulse in normal life.  At the same time, though no special acquaintance with the pathology of the subject will be laid claim to, frequent references will be made to the illusions of the insane.  Indeed, it will be found that the two groups of phenomena—­the illusions of the normal and of the abnormal condition—­are so similar, and pass into one another by such insensible gradations, that it is impossible to discuss the one apart from the other.  The view of illusion which will be adopted in this work is that it constitutes a kind of border-land between perfectly sane and vigorous mental life and dementia.

And here at once there forces itself on our attention the question, What exactly is to be understood by the term “illusion”?  In scientific works treating of the pathology of the subject, the word is confined to what are specially known as illusions of the senses, that is to say, to false or illusory perceptions.  And there is very good reason for this limitation, since such illusions of the senses are the most palpable and striking symptoms of mental disease.  In addition to this, it must be allowed that, to the ordinary reader, the term first of all calls up this same idea of a deception of the senses.

At the same time, popular usage has long since extended the term so as to include under it errors which do not counterfeit actual perceptions.  We commonly speak of a man being under an illusion respecting himself when he has a ridiculously exaggerated view of his own importance, and in a similar way of a person being in a state of illusion with respect to the past when, through frailty of memory, he pictures it quite otherwise than it is certainly known to have been.

It will be found, I think, that there is a very good reason for this popular extension of the term.  The errors just alluded to have this in common with illusions of sense, that they simulate the form of immediate or self-evident cognition.  An idea held respecting ourselves or respecting our past history does not depend on any other piece of knowledge; in other words, is not adopted as the result of a process of reasoning.  What I believe with reference to my past history, so far as I can myself recall it, I believe instantaneously and immediately, without the intervention of any premise or reason.  Similarly, our notions of ourselves are, for the most part, obtained apart from any process of inference.  The view which a man takes of his own character or claims on society he is popularly supposed to receive intuitively by a mere act of internal observation.  Such beliefs may not, indeed, have all the overpowering force which belongs to illusory perceptions, for the intuition of something by the senses is commonly looked on as the most immediate and irresistible kind of knowledge.  Still, they must be said to come very near illusions of sense in the degree of their self-evident certainty.