Final answer:
Louis Sullivan's famous assertion was that form follows function in architectural design, meaning that the design of a building should be determined by its intended function or purpose.
Step-by-step explanation:
Louis Sullivan's famous assertion was that form follows function in architectural design. This means that the design of a building should be determined by its intended function or purpose. Sullivan believed that the aesthetic and structural aspects of a building should be directly influenced by its function, rather than using ornamentation or decoration for the sake of appearance.
For example, in his design of the Wainwright Building in St. Louis, Sullivan prioritized the functional aspects of the building, such as providing efficient office spaces, while still incorporating artistic elements like the intricate terra cotta details on the facade.