The history of the ideas about the nature of sickness is epitomized in our attempts at a rational and comprehensive classification of diseases. For a classification to be not only intellectually acceptable but also practical and meaningful, some logical pattern should be followed in tracing the most plausible, though definitely arbitrary, boundary lines between the various subgroups of pathologic entities.
Meaningfulness—never an absolute value in itself—acquires in this context a remarkable volatility. This is displayed quite vividly with every attempt at including newly recognized conditions into a working classification. When the new addition is readily incorporated, the entire scheme gains in meaningfulness. This almost inevitably totters when the rigidity of the existing framework is revealed by its inability to accommodate a new finding. Thus classifications have a built-in obsolescence which is linked to the tide of advancing knowledge and which calls for periodic revisions.
It frequently happens that too little