A little consideration, however, will show that this is not an adequate definition of what is ordinarily understood by an illusion of sense.  There are special circumstances which are fitted to excite a momentary illusion in all minds.  The optical illusions due to the reflection and refraction of light are not peculiar to the individual, but arise in all minds under precisely similar external conditions.

It is plain that the illusoriness of a perception is in these cases determined in relation to the sense-impressions of other moments and situations, or to what are presumably better percepts than the present one.  Sometimes this involves an appeal from one sense to another.  Thus, there is the process of verification of sight by touch, for example, in the case of optical images, a mode of perception which, as we have seen, gives a more direct cognition of external quality.  Conversely, there may occasionally be a reference from touch to sight, when it is a question of discriminating two points lying very close to one another.  Finally, the same sense may correct itself, as when the illusion of the stereoscope is corrected by afterwards looking at the two separate pictures.

We may thus roughly define an illusion of perception as consisting in the formation of a quasi-percept which is peculiar to an individual, or which is contradicted by another and presumably more accurate percept.  Or, if we take the meaning of the word common to include both the universal as contrasted with the individual experience, and the permanent, constant, or average, as distinguished from the momentary and variable percept, we may still briefly describe an illusion of perception as a deviation from the common or collective experience.

Sources of Sense-Illusion.

Understanding sense-illusion in this way, let us glance back at the process of perception in its several stages or aspects, with the object of discovering what room occurs for illusion.

It appears at first as if the preliminary stages—­the reception, discrimination, and classification of an impression—­would not offer the slightest opening for error.  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.