  Le Poidevin (2007: 88–92) tentatively forwards an alternative hypothesis. As Aristotle noted, if you stare at a waterfall for a short period, and then turn your gaze to the bank beside it, you will see part of the bank (seemingly) start to move in an upwards direction. This phenomenon is commonly called ‘the waterfall illusion’ (or motion aftereffect), and the illusory motion is of an intriguing sort. You will not see a part of the bank detach itself from its surroundings and drift up towards the sky. Rather, you will see the contents within a fixed and immobile region of the bank – roughly the size of the waterfall you were staring at previously – losing their normal solidity and becoming fluid-like. Although these contents seems to be flowing upward, they do so without changing – at least in a clear and unambiguous way – their location with respect to the rest of the  bank.[8] As for an explanation of what is going on in such cases, Le Poidevin (following Richard Gregory) suggests that perhaps we can discern here the workings of two distinct neural mechanisms. One ‘registers what we might call “pure motion”, i.e., gives rise to the impression of motion without any associated sense of change of position. It is this system that is responsible for the sense of perceiving motion as happening now.’ (2007: 89) A second system, relying on short-term memory, tracks and compares the alterations in location over time. This second system is not concerned with telling us about presently occurring motions, rather it gives rise to the sense that objects have changed their positions relative to one another. Hence Le Poidevin’s proposal. Perhaps our ordinary experience of motion does, after all, consist of nothing but momentary static snapshots – in accord with Cinematic antirealism – but these momentary experiences seem dynamic thanks to the activation of the ‘pure motion’ mechanism in our visual system. These snapshots do not actually feature movement, but as the waterfall illusion illustrates, movement – at least in the form of change of position over time – is not required for the vivid impression of motion.