I do not mean that there is an observant mind behind and distinct from the sensation, and failing to observe it accurately through a kind of mental short-sightedness.

What I mean is that the usual psychical effect of the incoming nervous process may to some extent be counteracted by a powerful reaction of the centres.

In the course of our study of illusions, we shall learn that it is possible for the quality of an impression, as, for example, of a sensation of colour, to be appreciably modified when there is a strong tendency to regard it in one particular way.
Postponing the consideration of these, we may say that certain illusions appear clearly to take their start from an error in the process of classifying or identifying a present impression.

On the physical side, we may say that the first stages of the nervous process, the due excitation of the sensory centre in accordance with the form of the incoming stimulation and the central reaction involved in the recognition of the sensation, are incomplete.

These are so limited and comparatively unimportant a class, that it will be well to dispose of them at once.
_Confusion of the Sense-Impression._ The most interesting case of such an error is where the impression is unfamiliar and novel in character.