The right finger precedes and apprehends a group of points simultaneously (synthetic touch), the left finger follows somewhat more slowly and apprehends the single points successively (analytic touch). Both the synthetic and analytic impressions are united and referred to the same object. This method of procedure shows clearly that the spacial discrimination of tactual impressions is no more immediately given in this case than in the case where vision was present, but that here the improvements by means of which the finger that is used for analytic touch passes from point to point, play the same part as the accompanying visual ideas did in the normal cases with vision.
An idea of the extent and direction of these movements can arise only under the condition that every movement is accompanied by an inner tactual sensation (p. 46, 6). The assumption that these inner tactual sensations are immediately connected with an idea of the space passed through in the movement, would be highly improbable, for it would not only presuppose the existence of a connate perception of surrounding space and of the position of the subject in respect to the same (p. 103), but it would include another particular assumption. This is the assumption that inner and outer touch-sensations, although they are otherwise alike in quality and physiological substrata, still differ in that inner sensations give, along with the sensation, an image of the position of the subject and of the spacial arrangement of the immediate environment. This would really necessitate a return [p. 109] to the Platonic doctrine of the memory of innate ideas, for the sensations of movements arising from touch are here thought of as the mere external occasional causes for the revival of innate transcendental ideas of space.
7. Apart from its psychological improbability, such an hypothesis as that just mentioned can not be reconciled with the influence exercised by practice on the discrimination of local signs and of differences in movements. There is no other way except to attribute the rise of spacial ideas here, as in normal cases with vision (p. 106), to the combinations of the sensations themselves as presented in experience. These combinations consist in the fact that in passing from one outer tactual impression to another, any two sensations, a and b, with a certain difference in local signs, always have a corresponding inner touch-sensation, a, accompanying the movement; while two sensations, a and c, with a greater difference in local signs, have a more intense sensation of movement, g . For the blind there is always such a regular combination of inner and outer touch-sensations. From the strictly empirical point of view it can not be affirmed that either of these sensational systems, itself, brings the idea of spacial arrangement; we can only say that this arrangement results regularly from the combination of the two. On this basis the special ideas of the blind, arising, as they do, from external impressions, are defined as the product of the fusion of external tactual sensations and their qualitatively guided local signs, with internal tactual sensations, graded according to intensity. The external sensations with their attributes as determined by the external stimulus, are the predominating elements in this fusion. These push the local signs with their qualitative peculiarities, and the sensations of movement with their intensive attributes, so far into the background, that, like the overtones of a clang they [p. 110] can be perceived only when the attention is especially concentrated upon them. Spacial ideas from touch are, accordingly, due to a complete fusion. Their characteristic peculiarity, in contrast, for example, with intensive tonal fusions, is that the subordinate and supplementary elements are different in character, and at the same time related to one another according to definite laws.
They are different, for the local signs form a pure qualitative system, while the inner touch-sensations which accompany the movements of the tactual organs, form a series of intensifies. They are related, for the motor energy used in passing through an interval between two points, increases with the extent of the interval, that, in proportion to the qualitative difference between the local signs, there must also be an increase in the intensity of the sensations of movement.
8. The spacial arrangement of tactual impressions is thus the product of a twofold fusion. First, the subordinate elements fuse, in that the various qualities of the local sign system, which is spread out in two dimensions, are related to one another according to the grades of intensity of the sensations of movement. Secondly, the tactual impressions as determined by the external stimuli, fuse with the product of the first union. Of course, the two processes do not take place successively, but in one and the same act, for the local signs and movements must both be aroused by the external stimuli. Still, the external sensations vary with the nature of the objective stimulus, while the local signs and internal tactual sensations are subjective elements, whose mutual relations always remain the same even when the external impressions vary. This is the psychological condition for the constancy of attributes which we ascribe to space itself, in contrast wich [sic] the great changeableness of the qualitative attributes of objects in space. [p. 111]