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.
 I here touch on the distinction between the psychological and the philosophical view of perception, to be brought out more fully by-and-by.
 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.
 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.
 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.
 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.)
 This relation will be more fully discussed under the head of “Memory.”
 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.
 Such verbal suggestion, moreover, acting through a sense-impression, has something of that vividness of effect which belongs to all excitation of mental images by external stimuli.
 See Wundt, Physiologische Psychologie, p. 723.