his studies on conics, perspective and projective geometry. Enlightenment The Age of the Enlightenment witnessed considerable development in architectural theory on the European continent. New archaeological discoveries (such as those of Pompeii and Herculaneum) drove new interest in Classical art and architecture. Thus, the term neoclassicism, exemplified by the writings of Prussian art critic Johann Joachim Winkelmann, arose to designate 18th-century architecture, which looked to these new classical precedents for inspiration in building design. Major architectural theorists of the Enlightenment include Julien-David Leroy, Abbé Marc-Antoine Laugier, Giovanni Battista Piranesi, Robert Adam, James Stuart, Georg Friedrich Hegel and Nicholas Revett. 19th century A vibrant strain of Neoclassicism, inherited from Marc-Antoine Laugier's seminal Essai, provided the foundation for two generations of international activity around the core themes of classicism, primitivism and a "return to Nature." Reaction against the dominance of neoclassical architecture came to the fore in the 1820s with Augustus Pugin providing a moral and theoretical basis for Gothic Revival architecture, and in the 1840s John Ruskin developed this ethos. The American sculptor Horatio Greenough published the essay "'American Architecture" in August 1843, in which he rejected the imitation of old styles of buildings and outlined the functional relationship between architecture and decoration. These theories anticipated the development of Functionalism in modern architecture. Towards the end of the century, there occurred a blossoming of theoretical activity. In England, Ruskin's ideals underpinned the emergence of the Arts and Crafts movement exemplified by the writings of William Morris. This in turn formed the basis for Art Nouveau in the UK, exemplified by the work of Charles Rennie Mackintosh, and influenced the Vienna Secession. On the Continent, the theories of Viollet-le-Duc and Gottfried Semper provided the springboard for enormous vitality of thought dedicated to architectural innovation and the renovation of the notion of style. Semper in particular developed an international following, in Germany, England, Switzerland, Austria, Bohemia, France, Italy and the United States. The generation born during the middle-third of the 19th century was largely enthralled with the opportunities presented by Semper's combination of a breathtaking historical scope and a methodological granularity. In contrast to more recent, and thus "modern", thematically self-organized theoretical activities, this generation did not coalesce into a "movement." They did, however, seem to converge on Semper's use of the concept of Realismus, and they are thus labelled proponents of architectural realism. Among the most active Architectural Realists were: Georg Heuser,