On the surface, the theories of Jean Piaget and Sigmund Freud might seem to lie in different scientific universes. Piaget seems to be concerned almost wholly with intellectual matters, while Freud studies emotional mechanisms. Yet the differences are not really clear. It is widely understood that emotional behavior depends on unconscious machinery, but we do not so often recognize that ordinary intellectual thinking also depends on mechanisms that are equally hidden from introspection.
In any case, despite their differences, both these great psychologists asserted that every child proceeds through stages of mental development. And surely every parent notices how children sometimes seem to stay the same but at other times appear to change more rapidly. Rather than review particular theories of how children progress through stages, let's look at the concept of stage itself.
Why can't we grow by steady, smooth development?
I'll argue that nothing so complex as a human mind can grow, except in separate steps. One reason is that it is always dangerous to change a system that already works. Suppose you discover a new idea or way to think that seems useful enough to justify building more skills that depend on it. What happens if, later, it should turn out that this idea has a serious flaw? How could you restore your previous abilities? One way might be to maintain such complete records that you could undo all the changes that were made — but that wouldn't work if those changes had already made your quality of thought so poor that you couldn't recognize how poor it had become. A safer way would be to keep some older versions of your previous mind intact as you constructed each new version. Then you could regress to a previous stage in case the new one failed, and you could also use it to evaluate the performance of the new stage.
Another conservative strategy is never to let a new stage take control of actual behavior until there is evidence that it can outperform its predecessor. What would an outside observer see if a child employed this strategy? One would observe only plateaus, during which there were few apparent changes in behavior, followed by spurts of growth in which new capacities emerge suddenly. Yet that appearance would be illusory, since the actual times of development would occur within those silent periods. This scheme has the great advantage of permitting the child to continue to function during mental growth and, thus, maintain business during renovations. Each working version can hold still while new ones safely move ahead.
This applies to every large organization, not only to those involved in a child's development. Given a community that is already functioning, it is always dangerous to make more than a few changes at once. Each change is prone to have some harmful side effects on other systems that depend on it. Some of those side effects may not become apparent until so many of them have accumulated that the system has deteriorated past any point of turning back. Accordingly, it is better to stop from time to time to make inspections and repairs. The same is true for learning any complex skill; unless your goal is held unchanged for long enough, you won't have time enough to learn the skills required to accomplish it. It simply isn't practical to make minds grow by steady, smooth development.