THE PERCEPTION OF SPACE. (l.) 21

sign, that is, a feeling which, the first time it enters the mind, evokes from the native powers thereof a cognition of the thing that hitherto had lain dormant. In both cases, however, the sign is one thing, and the thing another. In the instance that now concerns us, the sign is a quality of feeling and the thing is a position. Now we have seen that the position of a point is not only revealed, but created, by the presence of other external points to which it stands in determinate relations. If the sign can by any machinery which it sets in motion evoke a consciousness either of the other points, or of the relations, or of both, it would seem to fulfil its function, and reveal to us the position we seek. But such a machinery is already familiar to us. It is neither more nor less than the law of habit in the nervous system. When any point of the sensitive surface has been frequently excited simultaneously with, or immediately before or after other points, and afterwards comes to be excited alone, there will be a tendency for its perceptive nerve-centre to irradiate into the nerve-centres of the other points. Subjectively considered, this is the same as if we said that the local-sign, the peculiar feeling, of the first point, when aroused, will suggest the feeling of the entire region with whose stimulation its own excitement has heen habi- tually associated. Take the case of the stomach. When the epigastrium is heavily pressed, when certain muscles contract, &c., the stomach is squeezed, and its peculiar local-sign awakes in consciousness simultaneously with the local-signs of the other squeezed parts. There is also a sensation of total vastness aroused by the combined irritation, and somewhers in this the stomach-feeling seems to lie. Suppose that later a pain arises in the stomach from some non-mechanical cause. It will be tinged by the gastric local-sign, and the nerve-centre supporting this latter feeling will excite the centre supporting the dermal and muscular feelings habitually associated with it when the excitement was mechanical. From the combination the same peculiar vastness will again arise. In a word, ' something ' in the stomach-sensation will ' remind ' us of a total space of which the diaphragmatic and epigastric sensations also form a part, or, to express it more briefly still, will suggest the neighbourhood of these latter organs. 1 1 Maybe the localisation of intracranial pain is itself clue to such asso- ciation as this of local-signs with each other, rather than to their quali- tative similarity in neighbouring parts (supra, p. 19) ; though it is con-