The nervous counterpart of the final stage of perception, the synthesis of the sensation and the mental representation, is not clearly ascertained. A sensation clearly resembles a mental image in quality. It is most obviously marked off from the image by its greater vividness or intensity. Agreeably to this view, it is now held by a number of eminent physiologists and psychologists that the nervous process underlying a sensation occupies the same central region as that which underlies the corresponding image. According to this theory, the two processes differ in their degree of energy only, this difference being connected with the fact that the former involves, while the latter does not involve, the peripheral region of the nervous system. Accepting this view as on the whole well founded, I shall speak of an ideational, or rather an imaginational; and a sensational nervous process, and not of an ideational and a sensational centre.[12] The special force that belongs to the representative element in a percept, as compared with that of a pure "perceptional" image,[13] is probably connected with the fact that, in the case of actual perception, the nervous process underlying the act of imaginative construction is organically united to the initial sensational process, of which indeed it may be regarded as a continuation. For the physical counterpart of the two stages in the interpretative part of perception, distinguished as the passive stage of preperception, and the active stage of perception proper, we may, in the absence of certain knowledge, fall back on the hypothesis put forward by Dr. J. Hughlings Jackson, in the articles in _Brain_ already referred to, namely, that the former answers to an action of the right hemisphere of the brain, the latter to a subsequent action of the left hemisphere. The expediting of the process of preperception in those cases where it has frequently been performed before, is clearly an illustration of the organic law that every function is improved by exercise. And the temporary disposition to perform the process due to recent imaginative activity, is explained at once on the physical side by the supposition that an actual perception and a perceptional image involve the activity of the same nervous tracts. For, assuming this to be the case, it follows, from a well-known organic law, that a recent excitation would leave a temporary disposition in these particular structures to resume that particular mode of activity.