Sullivan attributed the concept to Marcus Vitruvius Pollio, the Roman architect, engineer, and author, who first asserted in his book, "De architectura (On architecture)", that a structure must exhibit the three qualities of "firmitas, utilitas, venustas" – that is, it must be "solid, useful, beautiful." This credo, which placed the demands of practical use above aesthetics, later would be taken by influential designers to imply that decorative elements, which architects call "ornament", were superfluous in modern buildings, but Sullivan neither thought nor designed along such dogmatic lines during the peak of his career. While his buildings could be spare and crisp in their principal masses, he often punctuated their plain surfaces with eruptions of lush Art Nouveau or Celtic Revival decorations, usually cast in iron or terra cotta, and ranging from organic forms, such as vines and ivy, to more geometric designs and interlace, inspired by his Irish design heritage. Terra cotta is lighter and easier to work with than stone masonry. Sullivan used it in his architecture because it had a malleability that was appropriate for his ornament. Probably the most famous example of ornament used by Sullivan is the writhing green ironwork that covers the entrance canopies of the Carson Pirie Scott store on south State Street.