The performer tells his audience that he is about to do a certain thing, for example, take a number of animals out of a small box which is incapable of holding them.

The hearers, intent on what has been said, vividly represent to themselves the action described.

And in this way their attention becomes bribed, so to speak, beforehand, and fails to notice the inconspicuous movements which would at once clear up the mystery.
Similarly with respect to the illusions which overtake people at spiritualist _seances_.

The intensity of the expectation of a particular kind of object excludes calm attention to what really happens, and the slightest impressions which answer to signs of the object anticipated are instantly seized by the mind and worked up into illusory perceptions.
It is to be noted that even when the impression cannot be made to tally exactly with the expectation, the force of the latter often effects a grotesque confusion of the perception.

If, for example, a man goes into a familiar room in the dark in order to fetch something, and for a moment forgets the particular door by which he has entered, his definite expectation of finding things in a certain order may blend with the order of impressions experienced, producing for the moment a most comical illusion as to the actual state of things.
When the degree of expectation is unusually great, it may suffice to produce something like the counterfeit of a real sensation.