on. There is a "personal equation"
in perception as in belief--an amount of erroneous deviation from the
common average view of external things, which is the outcome of
individual temperament and habits of mind. Thus, a naturally timid man
will be in general disposed to see ugly and fearful objects where a
perfectly unbiased mind perceives nothing of the kind; and the forms
which these objects of dread will assume are determined by the character
of his past experience, and by the customary direction of his
imagination.

In perfectly healthy states of mind this influence of temperament and
mental habit on the perception of external objects is, of course, very
limited; it shows itself more distinctly, as we shall see, in modifying
the estimate of things in relation to the aesthetic and other feelings.
This applies to the mythical poetical way of looking at nature--a part
of our subject to which we shall have to return later on.

Passing now from the effect of such permanent dispositions, let us look
at the more striking results of temporary expectancy of mind.

When touching on the influence of such a temporary mental attitude in
the process of correct perception, I remarked that this readiness of
mind might assume an indefinite or a definite form. 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. Emotion is the great disturber of
all intellectual operations. It effects marvellous things, as we shall