Abstract
Illusions are important 'tools' in the study of perceptual processes. Their conception is typically linked to the notion of veridicality in a dual-world framework, in which we either see the macro physical world as it is (ecological approaches) or we derive a faithful representation (cognitive approaches) of it. Within such theoretical views, illusions are errors caused by inadequate sensory information (because of poor quality, insufficient quantity, contradictory, etc.). From a phenomenological stance, however, experiencing an illusion does not relate to the physical quality of the distal or proximal stimulus; rather, it depends on a comparison between the actual perception and what one believes should be perceived given the knowledge s/he has gained about the physical stimulus. Within such a framework, illusions are still considered of extreme importance in the study of the processes underpinning perception, but they are not conceived as errors. They represent instead a cognitive clash between actual perception and hypothesized perception based on some sort of comparison, thus also showing their potential as a tool for studying the underpinnings of cognitive processes.