Classifications have been used for centuries with the goal of cataloguing and searching large sets of objects. In the early days it was mainly books; lately it has become Web pages, pictures and any kind of electronic information items. Classifications describe their contents using natural language labels, an approach which has proved very effective in manual classification. However natural language labels show their limitations when one tries to automate the process, as they make it almost impossible to reason about classifications and their contents. In this paper we introduce the novel notion of Formal Classification, as a graph structure where labels are written in a logical concept language. The main property of Formal Classifications is that each node can be associated a normal form formula which univocally describes its contents. This in turn allows us to reduce document classification and query answering to fully automatic propositional reasoning.