Words and Things is one of the most difficult and controversial works of the French thinker. Foucault examines here those historically changing structures ("historical a priori") that determine the conditions for the possibility of opinions, theories or even sciences in each historical period, and calls them "epistems." Foucault contrasts "archaeology," which isolates these epistems, with historical knowledge of the cumulativist type, which describes certain opinions without clarifying the conditions of their possibility. The basic ordering principle within each system is the relation of "words" and "things."