It is evident that all indistinct impressions are liable to be wrongly classed.  Sensations answering to a given colour or form, are, when faint, easily confused with other sensations, and so an opening occurs for illusion.  Thus, the impressions received from distant objects are frequently misinterpreted, and, as we shall see by-and-by, it is in this region of hazy impression that imagination is wont to play its most startling pranks.

It is to be observed that the illusions arising from wrong classification will be more frequent in the case of those senses where discrimination is low.  Thus, it is much easier in a general way to confuse two sensations of smell than two sensations of colour.  Hence the great source of such errors is to be found in that mass of obscure sensation which is connected with the organic processes, as digestion, respiration, etc., together with those varying tactual and motor feelings, which result from what is called the subjective stimulation of the tactual nerves, and from changes in the position and condition of the muscles.  Lying commonly in what is known as the sub-conscious region of mind, undiscriminated, vague, and ill-defined, these sensations, when they come to be specially attended to, readily get misapprehended, and so lead to illusion, both in waking life and in sleep.  I shall have occasion to illustrate this later on.

With these sensations, the result of stimulations coming from remote parts of the organism, may be classed the ocular impressions which we receive in indirect vision.  When the eye is not fixed on an object, the impression, involving the activity of some-peripheral region of the retina, is comparatively indistinct.  This will be much more the case when the object lies at a distance for which the eye is not at the time accommodated.  And in these circumstances, when we happen to turn our attention to the impression, we easily misapprehend it, and so fall into illusion.  Thus, it has been remarked by Sir David Brewster, in his Letters on Natural Magic (letter vii.), that when looking through a window at some object beyond, we easily suppose a fly on the window-pane to be a larger object, as a bird, at a greater distance.[15]

While these cases of a confusion or a wrong classification of the sensation are pretty well made out, there are other illusions or quasi-illusions respecting which it is doubtful whether they should be brought under this head.  For example, it was found by Weber, that when the legs of a pair of compasses are at a certain small distance apart they will be felt as two by some parts of the tactual surface of the body, but only as one by other parts.  How are we to regard this discrepancy?  Must we say that in the latter case there are two sensations, only that, being so similar, they are confused one with another?  There seems some reason for so doing, in the fact that, by a repeated exercise of attention to the experiment, they may afterwards be recognized as two.