We've scarcely mentioned at all inside this book the kinds of quantities that could be measured — though surely brain cells use them all the time. For example, it seems quite likely that many of our agents employ quantitative schemes for summarizing evidence or establishing the strengths of connections. But I have said little about such matters because I suspect that such matters play diminished roles, the more we move toward higher-level operations of the mind. This is because whenever we're forced to compare magnitudes, we have to pay a heavy price: it tends to terminate what we call thinking.
Whenever we turn to measurements, we forfeit some uses of intellect. Currencies and magnitudes help us make comparisons only by concealing the differences among what they purport to represent.
By their nature, quantitative descriptions are so one-dimensional and featureless that they cannot help but conceal the structures that give rise to them. This is inescapable, since any act that makes two different things comparable must do it by deflecting our attention from their differences. Numbers themselves are the greatest masters of disguise because they perfectly conceal all traces of their origins. Add five and eight to make thirteen, and tell that answer to a friend: thirteen will be all your friend can know, since no amount of ingenious thought can ever show that it came from adding five and eight! It's much the same inside the head: quantitative judgments help us make decisions only by keeping us from thinking too much about the actual evidence.
No matter that such judgments have faults; you often have no choice but to choose. This happens when you can't stay where you are and must turn either right or left. Somewhere in some agencies, alternatives must be compared — and sometimes one can find no way except by using currencies. Then, various agents in your brain may turn to whatever quantities — chemical, electrical, or whatever — that happen to be available. Any substance or quantity whose availability is limited can be made to serve as a currency. But when we make our theories about how such systems work, we simply must remember not to make the easy mistake of confusing those quantities with their adopted functions and thus, for example, believing that certain drugs are inherently stimulating or depressing, or that certain foodstuffs are inherently more natural, or more healthy. Most of the properties of a currency are not inherent — but merely conventional.
In any case, we should never assume that the quality or character of a thought process depends directly on the nature of the circumstances that evoke it. There is no quality of sweetness inherent in sugar itself, which is a mere chemical. Its quality of sweetness is, in effect, a currency involved with certain agencies that are connected to sensors that detect the presence of sugar. Those agencies evolved that way because whenever we have hunger goals, it pays to recognize the taste of sugar as a sign of success — simply because sugar itself supplies energy, is easy to detect, and usually indicates the presence of other edible sources of nutrition. Similarly, inside our brains, many agencies have come to influence one another by controlling the amounts of various chemicals in much the way that many kinds of human transactions have come to use substances like candy, coins, or bags of salt — or banknotes backed by promises.