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).