1. Spacial and temporal ideas are immediately distinguished from intensive ideas by the fact that their parts are united, not in an arbitrarily variable, but in a definitely fixed order, so that when the order is thought of as changed the idea itself changes. Ideas with such a fixed arrangement are called in general extensive ideas.
Of the possible forms of extensive ideas, spacial ideas are distinguished by the fact that the fixed arrangement of the parts of a spacial compound holds only for the relation of the parts to one another, not for their relation to the ideating subject. This latter relation may be thought of as indefinitely varied. The objective independence of spacial compounds from the ideating subject is called the movability and torsibility of spacial compounds. The number of directions in which such movement and torsion may take place; is limited. They may all be reduced to three dimensions, in each of which it is possible to advance in two opposite directions. The number of directions in which the parts of a single compound may be arranged as well as the number in which various compounds may be arranged with reference to one another, is the same as the maximal number of directions in which movement and torsion are possible. This is what we call the three-dimensional character of space. A [p. 103] single spacial idea may, accordingly, be defined as a three dimensional compound whose parts are fixed in their location with regard to one another, but capable of indefinite variation in their location with regard to the ideating subject. This definition neglects, of course, the frequent changes in the arrangement of the parts, which occur in reality. When these changes take place, they are to be regarded as transitions from one idea to another.
This three-dimensional arrangement of spacial ideas must of necessity include one-dimensional and two-dimensional arrangements as special cases. In such cases, however, the wanting dimensions must always be added in thought as soon as the relation of the idea to the ideating subject is taken into account.
2. This relation to the ideating subject, which is really present in all spacial ideas, renders it from the first psychologically impossible that the arrangement of the elements in such an idea should be an original attribute of the elements themselves, analagous to the intensity or quality of sensations; it requires rather that this arrangement should result from the bringing together of these elements, and should arise from some new psychical conditions that come with this coexistence. If this is not admitted, it becomes necessary not only to attribute a spacial quality to every single sensation, but also to postulate for every sensation, however limited, a simultaneous idea of the whole of three-dimensional space in its location with regard to the ideating subject. This would lead to the acceptance of an a priori space-perception prior to all concrete sensations, which is not only contradictory to all our experiences as to the conditions for the rise and development of psychical compounds in general, but also contradictory to all our experiences as to the influences that affect spacial ideas in particular.
3. All spacial ideas are arrangements either of tactical [p.
104] or of visual sensations. Indirectly, through of connection of other sensations with either tactual or visual ideas, the spacial relation may be carried over to other sensations. In the cases of touch and sight, it is obvious that the extended surface of the peripheral sense-organs, and their equipment with organs of movement, which render possible a varying location of the impressions in regard to the ideating subject, are both favorable conditions for an extensive, spacial arrangement of the sensations.
The tactual sense is the earlier of the two here in question, for it appears earlier in the development of organisms and shows the structural relations in much coarser, but for that reason in many respects much plainer, form than the more delicately organized visual organ does. Still, it is to be noted that where vision is present, the spacial ideas from touch are greatly influenced by those from sight. A. SPACIAL TOUCH-IDEAS.
4. The simplest possible touch-idea is a single, approximiately punctiform impression on the skin. If such an impression is presented even when the eyes are turned away, there arises a definite idea of the place touched. Introspection shows that this idea, which is called the localization of the stimulus, under the normal condition where vision is present is not immediate, as we should expect it to be if the spacial quality were an original attribute of sensations, but that it depends upon a secondary, generally very obscure, visial idea of the region touched. Localization is, therefore, more exact near bounding lines of the touch-organs than on the uniform intervening surfaces, since these bounding lines are more prominent in the visual images. The arousal of a visual idea through the tactual impression, even when the eyes are turned away, is possible because every point of the organ of touch [p. 105] gives to the touch-sensation a peculiar qualitative coloring, which is independent of the quality of the external impression and is probably due to the character of the structure of the skin, which varies from point to point and is never exactly the same in two separate regions.
This local coloring is called the local sign of the sensation.