Illusion may arise in connection with either of these forms.

Let us illustrate both varieties, beginning with those which are due to a lasting mental disposition.
It is impossible here to specify all the causes of illusion residing in organized tendencies of the mind.

The whole past mental life, with its particular shade of experience, its ruling emotions, and its habitual direction of fancy, serves to give a particular colour to new impressions, and so to favour illusion.

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.