penguins and ostriches

"It's not 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 or 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 incapable of flight.

Unless we treat exceptions separately, they'll wreck all the generalizations we may try to make. 

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."