We here come on the puzzling question, How much in the character of the sensation must be regarded as the necessary result of the particular mode of nervous stimulation at the moment, together with the laws of sensibility, and how much must be put down to the reaction of the mind in the shape of attention and discrimination?  For our present purpose we may say that, whenever a deliberate effort of attention does not suffice to alter the character of a sensation, this may be pretty safely regarded as a net result of the nervous process, and any error arising may be referred to the later stages of the process of perception.  Thus, for example, the taking of the two points of a pair of compasses for one, where the closest attention does not discover the error, is best regarded as arising, not from a confusion of the sense-impression, but from a wrong interpretation of a sensation, occasioned by an overlooking of the limits of local discriminative sensibility.

Misinterpretation of the Sense-Impression.

Enough has been said, perhaps, about those errors of perception which have their root in the initial process of sensation.  We may now pass to the far more important class of illusions which are related to the later stages of perception, that is to say, the process of interpreting the sense-impression.  Speaking generally, one may describe an illusion of perception as a misinterpretation.  The wrong kind of interpretative mental image gets combined with the impression, or, if with Helmholtz we regard perception as a process of “unconscious inference,” we may say that these illusions involve an unconscious fallacious conclusion.  Or, looking at the physical side of the operation, it may be said that the central course taken by the nervous process does not correspond to the external relations of the moment.

As soon as we inspect these illusions of interpretation, we see that they fall into two divisions, according as they are connected with the process of suggestion, that is to say, the formation of the interpretative image so far as determined by links of association with the actual impression, or with an independent process of preperception as explained above.  Thus, for example, we fall into the illusion of hearing two voices when our shout is echoed back, just because the second auditory impression irresistibly calls up the image of a second shouter.  On the other hand, a man experiences the illusion of seeing spectres of familiar objects just after exciting his imagination over a ghost-story, because the mind is strongly predisposed to frame this kind of percept.  The first class of illusions arises from without, the sense-impression being the starting-point, and the process of preperception being controlled by this.  The second class arises rather from within, from an independent or spontaneous activity of the imagination.  In the one case the mind is comparatively passive; in the other it is active, energetically reacting on the impression, and impatiently anticipating the result of the normal process of preperception.  Hence I shall, for brevity’s sake, commonly speak of them as Passive and Active Illusions.[16]