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 THE PERCEPTION OF SPACE. (ll.) 205 feel the stability of the ends where the hinges and the lock are, and we seem to feel all three at once. 1 Such examples open up the whole subject of Extradi- tion, one of the most difficult problems which can occupy the space-philosopher. We shall see later in the special section on vision that the third dimension, or depth, has always been the stumbling-block of theorists. Here, how- ever, it behoves us to note that the seeming migration we have just studied, of a feeling from a joint to a finger-tip, with concomitant enlargement of size, seems to differ in no essential respect from those migrations beyond the skin with greater enlargement still. Closer examination will corrobo- rate this essential identity of the two cases, and the exami- nation will be much facilitated by recalling a few general principles at the start. We saw that all sensations are voluminous or contain the third dimension in a vague way. Projection, which is localisation of an impression at a deter- minate distance in this dimension, involves three factors : (1) feeling the extent of the dimension as a whole ; ' (2) dis- criminating a partial sensation within it ; (3) measuring the distance of that sensation from one of the extremes. It would appear therefore that, in the first instance at any rate, a sensation can be projected or extradited, only if it form part of a space--volume felt all at once, or in continuous succession. The mind in projecting would seem to identify its own position with that of one part of this volume, as a here, and detach from itself the other part, as a there. Now the centre the mind has thus chosen for its own felt habita- tion is undeniably sometimes within the head, sometimes within the throat or breast not a rigorously fixed spot there, but a region within which it seems to itself to move, 2 and from any portion of which it may send forth its various acts of attention. Extradition from either of these regions is the common law under which we perceive the whereabouts of the north star, of our own voice, of the contact of our teeth with each other, of the tip of our finger, the point of our cane on the ground, or a pain in our elbow-joint. The appearance of a feeling in the joint is as much a projection or a migra- tion as its appearance in the north star would be. Ampu- tations show how, owing to central excitement, limbs no longer existing are felt in their old site, or somewhat re- tracted. But the fact of extradition is the same when the 1 Cp. Lotze, Med. Psych., 428-433 ; Lipps, Grundtatsachen des Seelenlebens, 582. 2 The reader is reminded of the facts mentioned in sec. 1.