How can a hurt be canceled by a kiss? How can an insult add to injury? Why do we so often speak as though our wishes and desires were like forces, which increase one another's effects when they're aligned but cancel out when they're opposed? I'll argue that this is because, at every moment of our lives, we're forced to choose between alternatives we can't compare. Suppose, for example, that you must choose between two homes: one of them offers a mountain view; the other is closer to where you work. It really is a strange idea that two such unrelated things as nearness to work and beautiful scenery could be compared at all. But a person could instead assess that pleasant, restful view as worth a certain amount of travel time. Instead of comparing the items themselves, you could simply compare how much time they seem to be worth.
We turn to using quantities when we can't compare the qualities of things.
This way, for better or for worse, we often assign some magnitude or price to each alternative. That tactic helps to simplify our lives so much that virtually every social community works out its own communal measure-schemes — let's call them currencies — that let its people work and trade in harmony, even though each individual has somewhat different personal goals. The establishment of a currency can foster both competition and cooperation by providing us with peaceful ways to divide and apportion the things we have to share.
But who can set prices on things like time or measure the values of comfort and love? What makes our mental marketplaces work so well when emotional states seem so hard to compare? One reason is that no matter how different those mental conditions seem, they must all compete for certain limited resources — such as space, time, and energy — and these, to a rather large extent, are virtually interchangeable. For example, you'd end up with essentially the same result whether you measure things in terms of food or time — because it takes time to find food, and each amount of food helps you survive for some amount of time. Thus the value we place on each commodity constrains, to some extent, the values we'll assign to many other kinds of goods. Because there are so many such constraints, once a community sets up a currency, that currency takes on a life of its own, and soon we start to treat our wealth as though it were a genuine commodity, a real substance that we can use, save, lend, or waste.
In a similar way, a group of agencies inside the brain could exploit some amount to keep account of their transactions with one another. Indeed agencies need such techniques even more than people do, because they are less able to appreciate each other's concerns. But if agents had to pay their way, what might they use for currency? One family of agents might evolve ways to exploit their common access to some chemical that is available in limited quantities; another family of agents might contrive to use a quantity that doesn't actually exist at all, but whose amount is simply computed. I suspect that what we call the pleasure of success may be, in effect, the currency of some such scheme. To the extent that success is interchangeable with time or food or energy, it's useful to treat pleasure as equivalent to wealth.