 As Le Poidevin concedes, it is not obvious that the proposed explanation can be generalized: even if the existence of the envisaged twin-track neural systems could be established in the visual case, there is at present no reason to think that there are corresponding systems for all the other modes of experience, and as Realists will be swift to point out, change can be heard, felt, remembered and imagined, as well as  seen.[9] A second worry concerns the character of the ‘sense’ or ‘impression’ of motion delivered by the hypothesized pure motion system. These have to account for the very vivid appearances of motion in ordinary perceptions, not just the rather peculiar motions produced in waterfall-type illusions. How precisely do they do this? What is their character? There are only two obvious options. The impressions could themselves be sensory ingredients, of a visual kind, which are in some way suggestive of motion. Alternatively, they could be non-sensory: perhaps they are akin to judgements, or intuitive feelings (or convictions or beliefs), bearing a message along the lines of ‘that thing there is moving!’ The sensory option is problematic, for – as already noted in connection with Crick and Koch – it is not easy to see what feature of a still, static image could be sufficiently suggestive of motion. But the non-sensory option is also vulnerable to an objection. Realists will point out, once again, that perceived motion – both the ordinary kind, and the peculiar kind associated with motion aftereffect – exists on the level of sensory phenomena: it features prominently in our immediate experience. If this is right, the claim that motion-as-seen can be constituted of judgements or convictions, no matter how deep-seated or instinctive these might be, does not seem very plausible.