What controls the pace of mental growth? Although some aspects of development depend on external circumstances and others seem to happen only by chance, certain aspects of our growth seem almost to proceed relentlessly from stage to stage, as though those stages were predestined. This brings us back to asking why development proceeds in stages at all.
One reason a skill may grow in steps is that it needs prerequisites. You cannot start to build a house by placing its roof on top; first you have to build some walls. That's not an arbitrary rule; it's inherent in the enterprise. It is the same for mental skills; some processes cannot be learned until certain other processes become available. Many of Piaget's theories were based on his suspicion that certain concepts had prerequisites. For example, he argued that a child must possess ideas about which operations are reversible before that child can grow good concepts about how quantities are conserved. Hypotheses like these led Piaget to do his great experiments. But consider how easily those experiments could have been done a thousand years before; the only equipment they required were children, water, and various jars. Were Piaget's ideas prerequisites for conceiving those experiments?
To build a good Society-of-More, it simply would not be practical for a child to introduce those middle-level agents Appearance and History until some lower-level agents such as Tall, Thin, No Loss, and Reversible had become available. Before that stage, there would be nothing for those managers to do! To be sure, that isn't strictly true, just as one could start to construct a house with a roof, by using temporary scaffolding and later building the house's sides. We can never be absolutely sure of what a skill's prerequisites must be — and this will always complicate psychology.
The reason we know so little about how children's minds grow is that we can't observe the processes that are responsible. It could take several years to refine a new agency, and during that time, the child's behavior will be dominated by other processes in other agencies, which are themselves growing through their own, overlapping stages of development. A serious problem for the psychologist is that certain types of mental growth can never be directly observed at all. This applies, in particular, to those all-important B-brain processes with which we learn new ways to learn. Only the indirect products of this ever appear in the child's actual behavior, and even these may not become overt until long after that higher-level growth has occurred. Perhaps most difficult of all is detecting the development of suppressors and censors. See 27.2. It is hard enough to analyze what people do, but it is almost impossible to recognize the things they never do.
To make matters worse, many of the stages of development that we actually observe do not really exist. From time to time, each parent has the illusion that a child has suddenly changed, when this is only the result of not observing several smaller, real changes in the past. In such a case, if there exists a stage of growth, it is inside the parent's mind, and not in the child at all!