We will examine the effect of each kind in the production of illusion.
_Action of Sub-Expectation._ 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.