One axis of human sense-making runs from applied to derived; some things we understand by applying explanations to them, and others we understand by deriving explanations from them.

The component parts of water and the component parts of breakfast can both be described. Breakfast, however, exists only by definition — breakfast is the morning meal because we say it is, and even then, there are lots of caveats. A restaurant can advertise “Breakfast All Day!”; the same food at the same time can be breakfast or brunch; and so on.

Water is easier to explain than breakfast because water exists independently of any community — its existence can be derived by independent observers. Breakfast exists only because of and among people who say it does — it is an applied category, and therefore contingent on various ways of generating and enforcing shared understanding.

Because applied categories are social facts, they take more energy to define, and those definitions are less encompassing, less coherent, and less robust than for derived categories. And of course this is not a bifurcation but a spectrum. Imagine the difficulty of explaining each of three categories: “the New York Times,” “the media,” and “the liberal media”; or “the Republican Party,” “the conservative movement,” and “the vast right-wing conspiracy.” Each of those three categories is increasingly difficult to explain, because each is more of an applied category than the last, which is to say each requires more shared assumptions between sender and receiver.

Systems of classification frequently mix derived and applied categories. The more ambitious a system, in fact, the likelier it is to do so. If a significant number of the categories we use in our daily lives are applied, then large-scale classification systems will weaken with any of several changes, including especially an increase in the number or mix of users, an increase in the number of things to be classified, a decrease in coordination among the people doing the classifying, and the passage of time.

One strategy for improving classification schemes, or at least making them resistant to the inherent weakness of applied categories, is to reduce, possibly to zero, the number of universal assumptions or constraints within the system. This will minimize the energetic requirements of communicating and enforcing applied categories, while allowing the categories themselves to flow where and among whom they are considered useful or valid.

And this, of course, is exactly what tagging does. Much of the shared value, with little of the required force.