The practice, which began in the prehistoric era, has been used as a way of expressing culture for civilizations on all seven continents. For this reason, architecture is considered to be a form of art. Texts on architecture have been written since ancient time. The earliest surviving text on architectural theory is the 1st century AD treatise De architectura by the Roman architect Vitruvius, according to whom a good building embodies firmitas, utilitas, and venustas (durability, utility, and beauty). Centuries later, Leon Battista Alberti developed his ideas further, seeing beauty as an objective quality of buildings to be found in their proportions. Giorgio Vasari wrote Lives of the Most Excellent Painters, Sculptors, and Architects and put forward the idea of style in the Western arts in the 16th century. In the 19th century, Louis Sullivan declared that "form follows function". "Function" began to replace the classical "utility" and was understood to include not only practical but also aesthetic, psychological and cultural dimensions. The idea of sustainable architecture was introduced in the late 20th century.