What should one do with a law or rule that doesn't always work? We saw one way when we developed our uniframe for the Block-Arch. We simply kept changing it to fit each new example. But what if, after all that work, there still remain exceptions that don't fit?
The Exception Principle: It rarely pays to tamper with a rule that nearly always works. It's better just to complement it with an accumulation of specific exceptions.
All children learn that birds can fly and that animals that swim are fish. So what should they do when told that penguins and ostriches are birds that cannot fly, or that whales and porpoises are animals that swim but aren't fish? What should the children do with uniframes that no longer work so well? The exception principle says: Do not change them too hastily. We should never expect rules to be perfect but only to say what is typical. And if we try to modify each rule, to take each exception into account, our descriptions will become too cumbersome to use. It's not so bad to start with Birds can fly and later change it into Birds can fly, unless they are penguins or ostriches. But if you continue to seek perfection, your rules will turn into monstrosities:
Birds can fly, unless they are penguins and ostriches, or if they happen to be dead, or have broken wings, or are confined to cages, or have their feet stuck in cement, or have undergone experiences so dreadful as to render them psychologically incapable of flight.
Unless we treat exceptions separately, they'll wreck all the generalizations we may try to make. Consider why the commonsense idea of fish is so useful. It is an accumulation of general information about a class of things that have much in common: animals that live in the water, have a certain sort of streamlined shape, and move by wriggling their bodies and fanning the water with various finlike appendages. However, a biologist's idea of fish is very different, being more involved with the origins and internal mechanisms of those animals. This leads to emphasizing aspects less evident to the eye: if whales have lungs where trout have gills, they must be uniframed in different ways. Children are disturbed to hear that whales are not fish because they are usually more concerned with uses and appearances than with origins and mechanisms. They're more likely to want classifications like these:
What do those animals do? Where do they live? Are they easy to catch? Are they dangerous? Are they useful? What do they eat? How do they taste?
The power of ordinary words like fish comes from how we make them span so many meaning-worlds at once. However, in order to do this, we have to be able to tolerate many exceptions. We almost never find rules that have no exceptions — except in certain special, artificial worlds that we ourselves create by making up their rules and regulations to begin with. Artificial realms like mathematics and theology are built from the start to be devoid of interesting inconsistency. But we must be careful not to mistake our own inventions for natural phenomena we have discovered. To insist on perfect laws in real life is to risk not finding any laws at all. Only in the sciences, where every exception must be explained, does it make sense to pay that price.