Few of the schemes we've discussed would actually work if they were built exactly as they were described. Most of them would soon break down because virtually all their agents would become engaged into unconstrained activity. Suppose each typical agent tends to arouse several others. Then each of those would turn on several more — and each of those would turn on yet others; the activity would spread faster than a forest fire. But all that activity would accomplish nothing, since all those agents would interfere with each other and none of them could gain control of the resources they need. Indeed, this is more or less what happens in an attack of epilepsy.
Similar problems arise in every biological system. Each simple principle or mechanism must be controlled to operate within some limited range. Even little groups of genes embody schemes to regulate the quantities of proteins they cause to be manufactured inside every cell. We find the same pattern repeated on every scale. Every biological tissue, organ, and system is regulated by several kinds of control mechanisms, and wherever these fail we find disease. What normally protects our brains from such avalanches of activity? The cross- exclusion scheme is probably the most usual way to regulate the levels of activities within our agencies. But there are also several other frequently encountered schemes to prevent explosions.
Conservation: Force all activities to depend upon some substance or other kind of quantity of which only a certain amount is available. For example, we controlled our Snarc machine by setting a limit on the total electric current available to all the agents; this permitted only a few of them to be active at any particular moment. Negative Feedback: Supply a summary device that estimates the total activity in the agency and then broadcasts to that agency an inhibitory signal whose strength is in proportion to that total. This will tend to damp down incipient avalanches.
Censors and Suppressors: The conservation and feedback schemes tend to be indiscriminate. Later we'll discuss methods that are more sensitive and versatile in learning to recognize — and then to avoid — specific patterns of activity that have led to trouble in the past.
These methods are simple enough to be applied inside small societies, but they are not versatile enough to solve all the management difficulties that can arise in the more complex societies we need for learning to solve harder problems. Fortunately, systems built upon larger scales may be able to apply their own enhanced abilities to managing themselves — by formulating and solving their own self-regulation problems. In the next few sections we'll see how such capacities could grow in the course of several stages of development. Not all of this need happen independently inside each separate child's mind, because that child's family and cultural community can develop self-regulation schemes of great complexity. All human communities seem to work out policies for how their members ought to think, in forms that are thought of as common sense or as law, religion, or philosophy.