Final answer:
The motto of Louis Sullivan, famously applied in designs such as the Carson, Pirie, Scott Building, is "form follows function," focusing on functionality over ornate design.
Step-by-step explanation:
The motto of Louis Sullivan in designing the Carson, Pirie, Scott Building was "form follows function." This principle of modern architecture emphasizes that the shape of buildings should be primarily based upon their intended function or purpose. Sullivan's approach was a departure from ornate architectural styles of the past, focusing on simplicity, practicality, and eliminating unnecessary details that did not serve a functional purpose. His philosophy has had a lasting impact on the field of architecture, and his designs showcase the practical application of this concept.