First of all, then, our minds may at the particular moment be disposed to entertain any one of a vaguely circumscribed group of images.  Thus, to return to the example already referred to, when in Italy, we are in a state of readiness to frame any of the images that we have learnt to associate with this country.  We may not be distinctly anticipating any one kind of object, but are nevertheless in a condition of sub-expectation with reference to a large number of objects.  Accordingly, when an impression occurs which answers only very roughly to one of the associated images, there is a tendency to superimpose the image on the impression.  In this way illusion arises.  Thus, a man, when strolling in a cathedral, will be apt to take any kind of faint hollow sound for the soft tones of an organ.

The disposition to anticipate fact and reality in this way will be all the stronger if, as usually happens, the mental images thus lying ready for use have an emotional colouring.  Emotion is the great disturber of all intellectual operations.  It effects marvellous things, as we shall presently see, in the region of illusory belief, and its influence is very marked in the seemingly cooler region of external perception.  The effect of any emotional excitement appears to be to give a preternatural vividness and persistence to the ideas answering to it, that is to say, the ideas which are its excitants, or which are otherwise associated with it.  Owing to this circumstance, when the mind is under the temporary sway of any feeling, as, for example, fear, there will be a special readiness to interpret objects by help of images congruent with the emotion.  Thus, a man under the control of fear will be ready to see any kind of fear-inspiring object whenever there is any resemblance to such in the things actually present to his vision.  The state of awe which the surrounding circumstances of a spiritualist seance inspires produces a general readiness of mind to perceive what is strange, mysterious, and apparently miraculous.

It is worth noting, perhaps, that those delightful half-illusions which imitative art seeks to produce are greatly favoured by such a temporary attitude of the interpreting imagination.  In the theatre, for example, we are prepared for realizing the semblance of life that is to be unfolded before us.  We come knowing that what is to be performed aims at representing a real action or actual series of events.  We not improbably work ourselves into a slightly excited state in anticipation of such a representation.  More than this, as the play progresses, the realization of what has gone before produces a strong disposition to believe in the reality of what is to follow.  And this effect is proportionate to the degree of coherence and continuity in the action.  In this way, there is a cumulative effect on the mind.  If the action is good, the illusion, as every play-goer knows, is most complete towards the end.