When several objects move at once, it's hard to keep track of them all. This also seems to be the case in every other realm of thought; the more things we think about, the harder it is to pay attention to them all. We're forced to focus on a few while losing track of all the rest. What causes these phenomena? I'll argue that they're aspects of the processes we use to control our short-term memories. These skills develop over time; an adult can do things with memories that infants cannot do at all, such as remembering details of an action's purpose and trajectory, and how various obstacles were overcome. An infant, though, can barely keep track of what it's holding in one hand and is likely to forget what's in its other hand.
How does memory-control begin? Perhaps our infants first acquire control over a single pronome, which gives them the ability to keep in mind a temporary polyneme. This amounts to being able to maintain only a single object of attention; let's call it IT. Now even the ability to keep track of a single IT requires the development of certain skills of memory-control, for it takes the normal infant several months to become able to tolerate even a small interruption without losing its previous focus of interest.
One kind of interruption comes, for example, when watching a ball that happens to roll behind a box. To a very young infant, that IT will simply disappear from mind. An older infant will remember IT and expect the ball soon to reappear; we can see this in the way the older infant's eyes look toward the far side of the box. If the ball does not soon reappear, the older child will actively reach around the box for it, which shows that the child has maintained some sort of representation of IT. Another variety of interruption can come from inside the child's own mind, from refocusing on the same object, but at a different level of detail. For example, when a young child concentrates upon a doll's shoe, it may forget its original concern with the doll itself. Later, that concern with the shoe may be replaced, in turn, when the baby becomes occupied with the end of the shoelace.
But what's an IT? The ability to focus attention could start with some machinery for keeping track of simple polynemes for object-things. In later stages, an IT could represent more complex processes or scripts that keep track of entire Trans-actions with their various pronomes for Objects, Origins, Destinations, Obstacles, Trajectories, and Purposes. Eventually our ITs develop into complex systems of machinery that represent the things that are on one's mind at the moment. In later life, we become more able to maintain several ITs at once. This enables us to construct comparisons, predictions, and imaginary plans, and to begin to construct explanations in terms of chains of causes and reasons.