This part of the mechanism of perception seems to work so regularly and so smoothly that one can hardly conceive a fault in the process.

Nevertheless, a little consideration will show that even here all does not go on with unerring precision.
Let us suppose that the very first step is wanting--distinct attention to an impression.

It is easy to see that this will favour illusion by leading to a confusion of the impression.

Thus the timid man will more readily fall into the illusion of ghost-seeing than a cool-headed observant man, because he is less attentive to the actual impression of the moment.

This inattention to the sense-impression will be found to be a great co-operating factor in the production of illusions.
But if the sensation is properly attended to, can there be error through a misapprehension of what is actually in the mind at the moment?
To say that there can may sound paradoxical, and yet in a sense this is demonstrable.