Perhaps it means that it is impossible to create a rule that applies to a category without exception, real life is much more nuanced than that. So if you think

Perhaps it means that it is impossible to create a rule that applies to a category without exception, real life is much more nuanced than that. So if you think you have such a rule, that is always valid, as people often do, we know it's a bad rule. A rule only becomes a good rule once the exception is found because that means it is a rule that has a hope of succeeding in an ever evolving real life.