[27] I do not need here to go into the question whether, as Johannes Mueller assumed, this is an original attribute of nerve-structure, or whether, as Wundt suggests, it is due simply to the fact that certain kinds of nervous fibre have, in the course of evolution, been slowly adapted to one kind of stimulus.

[28] I here refer to what is commonly supposed to be the vague innate difference of sensation according to the local origin, before this is rendered precise, and added to by experience and association.

[29] The illusory character of this simple mode of perception is seen best, perhaps, in the curious habit into which we fall of referring a sensation of contact or discomfort to the edge of the teeth, the hair, and the other insentient structures, and even to anything customarily attached to the sentient surface, as dress, a pen, graving tool, etc.  On these curious illusions, see Lotze, Mikrokosmus, third edit., vol. ii. p. 202, etc.; Taine, De l’Intelligence, tom. ii. p. 83, et seq.

[30] Quoted by G.H.  Lewes, Problems of Life and Mind, third series, p. 335.  These illusions are supposed to involve an excitation of the nerve-fibres (whether sensory or motor) which run to the muscles and yield the so-called muscular sensations.

[31] It is brought out by Griesinger (loc. cit.) and the other writers on the pathology of illusion already quoted, that in the case of subjective sensations of touch, taste, and smell, no sharp line can be drawn between illusion and hallucination.

[32] For a fuller account of these pathological disturbances of sensibility, see Griesinger; also Dr. A. Mayer, Die Sinnestaeuschungen.

[33] Helmholtz, op. cit., p. 600, et seq. These facts seem to point to the conclusion that at least some of the feelings by which we know that we are expending muscular energy are connected with the initial stage of the outgoing nervous process in the motor centres.  In other pathological conditions the sense of weight by the muscles of the arms is similarly confused.

[34] Wundt (Physiologische Psychologie, p. 653) would exclude from illusions all those errors of sense-perception which have their foundation in the normal structure and function of the organs of sense.  Thus, he would exclude the effects of colour-contrast, e.g. the apparent modification of two colours in, juxtaposition towards their common boundary, which probably arises (according to E. Hering) from some mutual influence of the temporary state of activity of adjacent retinal elements.  To me, however, these appear to be illusions, since they may be brought under the head of wrong interpretations of sense-impressions.  When we see a grey patch as rose-red, as though it were so independently of the action of the complementary light previously or simultaneously, that is to say, as though it would appear rose-red to an eye independently of this action, we surely misinterpret.