§ 1. Spatial Perception of the Blind. — The existence of blind persons enables us to study touchspace dissevered from sightspace. But it is essential for this that the blind persons should either be blind from their birth, or have lost their sight in the first year of their lives; and also that they should retain no trace of sensibility to light or colour.* Those who have become blind in their fourth year translate their tactile impressions into visual imagery as we ourselves do in the dark.

* The facts adduced in this section are almost entirely due to Theodor Heller's most valuable "Studien zur BlindenPsychologie" in the Philo8ophische Studien, xi., 1895, pp. 226, 406, 531.

The chief instrument used by the blind in perceiving the shape and size of objects is the hand, or rather the two hands. These are used in a twofold way. (1) The hand, either open or closed, may touch simultaneously the parts of the object. This may be called passive touch, because it does not involve active movement from one part of an object to another. It may also be called synthetic touch, because it yields a total simultaneous impression of all or many parts of the object. (2) A portion of the hand, such as the fingertips, may explore the parts and contours of the object by gradually moving over them. This may be called active touch, because it essentially consists in active movement. It may also be called analytic touch, because it analyses or breaks up into a series of successive impressions what synthetic touch presents as a simultaneous whole. Now the main lesson that we learn from study of the blind is that all development in the definiteness of the perception of spatial order is essentially due to the intimate union and cooperation of synthetic and analytic touch. It must be understood that the observations and experiments on which we rely are all made on blind persons who have already acquired considerable experience. Their spatial perception is therefore at the outset developed in a large measure. What we can observe therefore is only the process by which greater precision and accuracy are acquired. It is fortunate for psychological purposes that spatial perception by touch does not reach full maturity with nearly the same rapidity as spatial perception by sight. Hence, even in the adult blind, it is possible to observe it in the process of growth. The first question with which we have to deal is, What information concerning shape and other spatial determinations is conveyed by synthetic touch apart from analytic? Of course, we cannot bring synthetic touch into play in absolute severance from analytic, for the blind have already had considerable experience in the exploration of objects and especially of their own bodies. In the case of simple and familiar things which they have already often explored by active touch, they can at once recognise shape, size, etc., by merely passive contact. But when objects are presented to them with which they are quite unfamiliar, it is found that for precise apprehension analytic touch must be combined with synthetic. Synthetic touch alone without the aid of previous experience yields at the most a general and schematic total impression. For instance, they can tell whether the object is round or angular, and whether it is regular or irregular. But for more precise determination of its shape, analytic movements are required. It is particularly noteworthy that the blind are almost incapable of confining themselves to purely synthetic touch when the object is at all unfamiliar. Involuntary twitchings of the hand occur which they find it difficult or impossible to suppress.

In the active exploration of objects there is a great difference in the method of procedure in different persons, and in different stages of development of the same person. The more highly the spatial perception has been developed, the more systematic and appropriate are the movements and their combination. At the highest stage the blind use a plan of procedure identical in its main features in different individuals. This is sometimes acquired in early childhood where the conditions are favourable. If the blind have to work with their hands, they always acquire the power of apprehending simple spatial relations. On the other hand, adults of otherwise good intelligence, who have not been compelled to acquire control over objects by pressure of practical needs, often show great helplessness, and do not appear to have any interest in spatial relations. In such cases, a special education of perceptual activity is required for adequate apprehension of the shape of objects. As education advances, the blind person becomes more and more capable of determining the size and shape of objects presented to him. At the same time, his active movements of exploration show a more and more systematic and purposeful character.