[1] A history of the distinction is given by Brierre de Boismont, in his work _On Illusions_ (translated by R. T. Hulme, 1859). He says that Arnold (1806) first defined hallucination, and distinguished it from illusion. Esquirol, in his work, _Des Maladies Mentales_ (1838), may be said to have fixed the distinction. (See Hunt's translation, 1845, p. 111.) [2] This fact has been fully recognized by writers on the pathology of the subject; for example, Griesinger, _Mental Pathology and Therapeutics_ (London, 1867), p. 84; Baillarger, article, "Des Hallucinations," in the _Memoires de l'Academie Royale de Medecine_, tom. xii. p. 273, etc; Wundt, _Physiologische Psychologie_, p. 653. [3] I here touch on the distinction between the psychological and the philosophical view of perception, to be brought out more fully by-and-by. [4] It might even be urged that the order here adopted is scientifically the best, since sense-perception is the earliest form of knowledge, introspected facts being known only in relation to perceived facts. But if the mind's knowledge of its own states is thus later in time, it is earlier in the logical order, that is to say, it is the most strictly presentative form of knowledge. [5] Here and elsewhere I use the word "impression" for the whole complex of sensation which is present at the moment. It may, perhaps, not be unnecessary to add that, in employing this term, I am making no assumption about the independent existence of external objects. [6] Psychological usage has now pretty well substituted the term "image" for "idea," in order to indicate an individual (as distinguished from a general) representation of a sensation or percept. It might, perhaps, be desirable to go further in this process of differentiating language, and to distinguish between a sensational image, _e.g._ the representation of a colour, and a perceptional image, as the representation of a coloured object. It may be well to add that, in speaking of a fusion of an image and a sensation, I do not mean that the former exists apart for a single instant. The term "fusion" is used figuratively to describe the union of the two sides or aspects of a complete percept. [7] This impulse to fill in visual elements not actually present is strikingly illustrated in people's difficulty in recognizing the gap in the field of vision answering to the insensitive "blind" spot on the retina. (See Helmholtz, _Physiologische Optik_, p. 573, _et seq._) [8] This relation will be more fully discussed under the head of "Memory." [9] I adopt this distinction from Dr. J. Hughlings Jackson. See his articles, "On Affections of Speech from Diseases of the Brain," in _Brain_, Nos. iii. and vii. The second stage might conveniently be named apperception, but for the special philosophical associations of the term: _Problems of Life and Mind_, third series, p. 107. This writer employs the word "preperception" to denote this effect of previous perception.