  Equally unremarkable is the fact that crawling infants distinguish 1 cracker from 2 and 2 from 3. This discrimination is beyond the abilities of the posited analog approximative systems. But it suggests that there is another system in the child that is in a limited way sensitive to number. This system—which seems more tied to attention—is the subject of current research (see, for instance, Pylyshyn 2007). Animals need to keep track of changing elements in their immediate environment. One idea under investigation is that there is a psychological subsystem system that ‘tags’ elements in a perceptual array and keeps track of them by assigning properties to the tag. Without such a system, we would lack the ability to re-identify changing elements from one moment to the next. Such a system therefore seems to be a prerequisite for any perception of a world/scene, as involving things that are moving and changing. It is difficult to see how an animal that does not track in this way could learn to do so (although the ability might grow).