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.