ILLUSION, in psychology and in epis temology, a perception which fails to reveal the true character of an object perceived. The word illusion is used in three different ways : (a) The mental construction, on the basis of data which are real in their own proper sphere, of a psychal object, which an illuded person ac cepts as real, but which is unreal or fantastic; (b) a mental object thus constructed; (c) the general mental acceptance and substitution of what is unreal for real.
In short, an illusion is an error. An error is nothing more than a psychological condition of blindness to the truth. By truth may be un derstood an inner assimilation of reality. The number of errors is unlimited, truth alone is one. When in psychological literature the word illusion occurs, it conveys one of the two ideas which receive definition respectively in defi nitions a and b; the notion conveyed in above occurs less frequently. Illusion is thus any mis taken mental construction which has reliable data as its point of departure. It always means a misinterpretation of the significance of a par ticular sensory impression. A sensory impres sion is the effect of an event upon a sentient being. But illusions occur in connection with not only the senses of taste, smell, touch, hear ing and seeing, but also with certain other and more obscure modes of consciousness. In truth it must be said that the senses represent reality neither wrongly nor rightly. All that science can say about the sense-organs in this connection is that under different circumstances they produce different sensations and percep tions. So that the term sense-illusion is a very misleading expression. The circumstances which condition sensation are extremely vari ous in character. They are partly external, be ing inherent in the objects, partly internal, being inherent in the sensory organs, and partly in terior, being confined in their activity within the central organs ; and therefore, it can some times appear, when notice is taken of only the external circumstances, as if an organ acted dif ferently under identical conditions. Thus it became customary to call the unusual effects sense-deceptions or sense-illusions. Consult Ernst Mach, die Abhavgigkeit der Netz hautstellen von einander' (in the scrift fiir Psychiatrie,) Leipzig and Neuwied 1868). Indeed there are no such things as il lusions of sense at all. Objects of sense ex perience, even when they occur in dreams, are the most indubitably real objects known. The truth is, that that which makes us call such ob jects unreal in dreams is merely the unusual connection in which they stand with other ob jects of sense. Objects of sense are called real when they have the relation with other objects of sense which experience has led us to regard as normal. Failing this, we call them illusions. But what is deceptive or illusory is only the in ferences to which they give rise. In themselves, they are every bit as real as the experienced ob jects of waking life. And conversely, sensible objects of waking life must not be assumed to have any more Intrinsic reality than sensible objects of dreams. It is only by some reality not merely sensible that dreams can be con demned. Illusion thus is a broad term. It cov ers all those errors of the logical operations as well which are not of the persistence or of the indirect delimitation, in any system of beliefs, which characterize delusions.
In mental pathology it is necessary to dis tinguish illusion from delusion (just used) and hallucination. In common parlance these terms are often confused, and many use them as if strictly synonymous. But this is not so among writers in psychiatry, for in this science these three terms are very clearly differentiated. And it is highly necessary that these distinctions should be observed, because these terms stand for very different phenomena in mental disease—phenomena of different value and im portance, and each with its own special signifi cance as to a patient's condition. An illusion
is distinguished from an 'hallucination in a two fold manner. In an hallucination the trust worthy data are absent; that is, the determining in the construction are either purely imaginative or of organic origin. Both these .cases manifest themselves on the border, or over the border, of the pathological. Halluci nation is narrower in application than the word illusion. It is confined to the perceptual— to objects of sense in contrast with the delusion of the higher mental processes. The limits of the three terms among themselves is however largely practical. The central processes of illusion and of hallucination, arising from organic causes, are one and the same. In most cases of either, the influence of the delusional elements are due to the earlier condition of the mind — notably, to emotional states. The cases of so-called pure sense-illusion, on the other hand, are really cases of perception, not of il lusion at all ; since they are normal, constant and (contrary to the vulgar opinion) common to all individuals, and also exposed by resort to tests outside of the sphere of the particular sense concerned. Still a distinction between the usual and normal sense-deceptions and the ab normal and unusual false interpretations of sen sory impressions and stimuli is very desirable. Some misinterpreting mental attitudes or fac tor seems necessary to the formation of the ordinary illusion. In the abnormal type of illu sion its etiology and nature is not much dis tinguished from hallucination. This fact is il lustrated by the illusions of the insane. In the case of the insane, it is very difficult to decide whether there exists an illusion or pure halluci nation, correlated with sensory and bodily ele ments. Ordinarily, when the mind is acting in a perfectly normal way and there is nothing to confuse its impressions or to obstruct its proper perceptive powers, a sensation, whether of the eye, ear, taste or skin is conveyed to the con sciousness and is recognized correctly. Thus a sound, as the note of a bird or the voice of a friend; or the sight of some object, as of an animal moving in the distance, is properly con veyed to the intelligence and is properly recog nized. But this process may be interfered with in various ways. Even the normal mind may make mistakes in the interpretation of sensa tions, and still more so may the mind that is impaired. It thus happens that all illusions are not necessarily evidences of a diseased mind; this faulty action of the senses, or of the per ception, may and often does happen in the cases of persons of sound mind. For instance, a per son walking along a path in the dusk may mis take a bush for an animal; he simply misinter prets an object and this object is not imaginary but has a real existence. So, too, the mirage which sometimes appears at sea, might lead a perfectly ignorant person to maintain that he saw a ship inverted sailing in the sky. These are instances of illusions in which the normal mind may be temporarily deceived. But the normal mind tends to correct its illusions, and this it does sooner or later according to its op portunity and its degree of knowledge. In this respect the insane mind differs from it; the il lusions of the insane are usually firmly believed in; there is no spontaneous tendency on the part of the insane mind to correct its illusions. On the contrary, they are firmly embraced and are often made the starting point of a train of associated morbid ideas, whereby the mental disorder is all the more confirmed.