Why do angry people act as though some measure of aggression must be spent and, when no proper object lies in reach, strike out and damage harmless things? It almost seems as though our feelings can accumulate like fluids bottled up inside. In earlier times, some scientists identified these quantities with substances like bile and blood. No one believes those theories now — yet still we often speak of having mental energy and momentum or of succumbing to depletion or inertia. Do mental quantities really exist within the mind? If so, how are they made and stored, brought forth and then spent? And what are their relations to the quantities and magnitudes we read about in technical books? The answer is that words like energy and force are not used with much precision in everyday psychology. They still have the connotations that they carried several centuries ago, when they referred to commonsense ideas about vitality. Then, energy referred to vigor of action and expression, and force referred to the binding strength of a commitment or to the fighting strength of an army.
Modern scientists use a concept of energy that, though narrower and more precise, not only explains more perfectly why engines stop when they run out of fuel, but also applies to our bodies as well: each of the cells of which we're made, including those inside the brain, requires some chemical energy in the form of food and oxygen. Accordingly, the body as a whole can do only a limited amount of physical work before it needs another meal. Now many people naively assume that our higher-level mental processes have similar requirements and that they need some second form of fuel — a mythical form of mental energy — to keep from becoming bored or mentally exhausted. And yet that simply isn't true! If each of Builder's agents has physical energy enough to do its work, then Builder — as an agency — needs nothing more to do its work. Builder, after all, is but a name for a certain assembly of agents. It can't require anything its separate agents do not need.
Machines and brains require ordinary energy to do their jobs — and need no other, mental forms of energy. Causality is quite enough to keep them working toward their goals.
But if our higher-level processes require no extra quantities like fuels or energies, what makes it seem to us as though they do? Why do so many people talk about their levels of mental or emotional energy? Why do tedious and boring occupations make us feel run down? We all experience so many such phenomena that we cannot help thinking our minds depend on many kinds of mental quantities — yet scientists apparently have shown that no such quantities exist. How can we explain this? It is not enough to say, simply, that these phenomena are illusions; we must understand why the illusions appear and, if possible, determine what functions they serve. The next few sections show how various illusions of mental force and energy evolve as convenient ways for mental agencies to regulate their transactions, much as many human communities have discovered how to use money.