This suggestion, taking the form of a vivid mental image, dominates and overpowers the passive impression.

Thus, in Meyer's experiment, the mind is possessed by the supposition that we are looking at the grey spot through a greenish medium.

So in the case of the distant viaduct, we are under the mastery of the idea that what we see in the distance is a red brick structure.
Once more, in the instance of looking at the picture, the spectator's imagination is enchained by the vivid representation of the object for which the picture stands, as the marble ruins in the moonlight or the Bedouin in the desert.
It may be well to add that this mental uncertainty as to the exact nature of a present impression is necessitated by the very conditions of accurate perception.

If, as I have said, all recognition takes place by overlooking points of diversity, the mind must, in course of time, acquire a habit of not attending to the exact quality of sense-impressions in all cases where the interpretation seems plain and obvious.

Or, to use Helmholtz's words, our sensations are, in a general way, of interest to us only as signs of things, and if we are sure of the thing, we readily overlook the precise nature of the impression.