For example, it could involve experiencing things moving and changing position outside of the centre of the visual field but as not moving at the centre. Or it could involve something more complex. The Waterfall Illusion is philosophically interesting for a number of reasons. Evolution, comparative morphology, and identification of the eumaeine butterfly genus Rekoa Kaye Lycaenidae: Theclinae.
In the passages we have quoted above from Aristotle and Addams the effect was simply described as involving movement expereinced in the oppositive direction to the previusly seen moving stimulus. This has now been verified experimentally by Wright and Johnson According to the sense-data theory, in veridical perception, illusion and hallucination, one is directly aware of some mental object a sense-datum that has the properties it appears to have - and in vitue of so doing, when the right conditions obtain for percpetion, one can come to see the external world indirectly in virtue of directly seeing sense-data.