Étienne-Louis Boullée’s Architecture, Essay on Art advances one of the most forceful theoretical claims of Enlightenment architecture: building is not merely the technical art of construction, but the poetic art of producing ideas through form. Boullée begins by challenging the reduction of architecture to Vitruvian utility and structural competence, insisting that the architect must study nature, sensation and the expressive power of volumes if architecture is to move the human mind. His argument depends on architectural character, the capacity of a building to declare its purpose, moral status and emotional charge through proportion, mass, light, shadow and disposition; a theatre should communicate pleasure, a palace dignity, a basilica majesty, a prison terror, and a monument civic grandeur. The essay’s most decisive conceptual case lies in Boullée’s theory of simple geometric bodies, especially the sphere, cube and pyramid, which he regards as uniquely capable of producing unity, clarity and sublimity because they are immediately intelligible to the eye and capable of overwhelming the imagination . His architectural method therefore transforms geometry into affect: vast surfaces, severe symmetry, controlled daylight, darkness and scale become instruments for awakening reverence, melancholy, joy or awe. The case of public monuments is especially revealing, because Boullée imagines architecture as a civic pedagogy, able to honour sovereigns, justice, nation and collective memory not through ornament alone but through an intensified correspondence between function and emotion. This does not make his architecture coldly abstract; on the contrary, his abstraction seeks maximum sensuous and moral force. Boullée’s definitive contribution is thus sublime composition: architecture becomes an art of intellectual drama, in which elementary forms are enlarged beyond ordinary habit so that buildings speak directly to the passions, transforming space into a theatre of reason, nature and public imagination.