Illusions : a psychological study.

  • Sully, James, 1842-1923.
Date:
1905
Catalogue details

Licence: In copyright

Credit: Illusions : a psychological study. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    58/416 (page 38)
    It is plain that the illusoriness of a perception is in these cases determined in relation to the sense- impress ions 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 veri- fication 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 percep- tion as consisting in the formation of a quasi-percept which is peculiar to an individual, or which is con- tradicted 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 mo- mentary 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.