Neoclassicism is a group of diverse buildings and architects from 1750-1850 (roughly) which share a particular attitude about the expressive potential of architecture, rather than any identifiable physical characteristic. The international movement was set in motion about 1750 by the following three occurrences: Abbé Laugier's Essay on Architecture (1753) which professed the authority of nature, a new attitude towards the past inspired by the documentation of ancient sites such as Pompeii, and the notion that architecture had a didactic function.
The Panthéon in Paris, designed by Jacques Germain Soufflot in 1731 gave lucid expression to the intellectual and visual ideas of the theorists. It was secularized by the French revolutionary government and became a monument for the burial of national heroes. Like Diderot's Encyclopedia, the Panthéon attempted to summarize the knowledge of the day in light of a ration explanation of the universe and an increasingly skeptical attitude to religion. On an urban scale, these ideas were articulated in the design of an ideal town, the saltworks at Arc and Senans at Chaux, by Claude-Nicolas Ledoux in the 1770s. The individual buildings at Chaux illustrate the idea of articulating a structure's function, or "architecture parlante." Etienne Louis Boullée's revolutionary architecture existed only on paper, and featured regular solids depicted in brooding, romantic ways. Although his ideas may seem far from examples like the Panthéon in their visual language, Boullée's architecture was also based on knowledge and reason.
This marriage of classical forms and emotional expression was carried to its extreme in the work of British architect John Soane, who frequently showed his buildings in a state of ruins or as a sequence of perspectives. His own home, now a museum, is evidence of his careful study of the past and his personal, nearly whimsical version of Neoclassicism.
Thomas Jefferson's particularly American take on the movement is best seen in the planning of the University of Virginia, where architecture symbolized his liberal views on education and democracy. Like other neoclassical ensembles, the university can be read as a catalogue of architectural elements. The 1792 plan for Washington DC by Pierre Charles L'Enfant is the best example of a neoclassical city, not necessarily in its plan (which was baroque), but rather in its spacing of monumental buildings. Like neoclassical architecture, it was intended to express larger ideas about history, nature, and the universe, and to underline the role of the architect as the purveyor of such knowledge.