Architecture is the art and technique of designing and building, as distinguished from the skills associated with construction. It is both the process and the product of sketching, conceiving, planning, designing, and constructing buildings or other structures. The term comes from Latin architectura; from Ancient Greek ἀρχιτέκτων (arkhitéktōn) 'architect'; from ἀρχι- (arkhi-) 'chief', and τέκτων (téktōn) 'creator'. Architectural works, in the material form of buildings, are often perceived as cultural symbols and as works of art. Historical civilisations are often identified with their surviving architectural achievements. The practice, which began in the prehistoric era, has been used as a way of expressing culture by 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 times. The earliest surviving text on architectural theories 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. 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. Architecture began as rural, oral vernacular architecture that developed from trial and error to successful replication. Ancient urban architecture was preoccupied with building religious structures and buildings symbolizing the political power of rulers until Greek and Roman architecture shifted focus to civic virtues. Indian and Chinese architecture influenced forms all over Asia and Buddhist architecture in particular took diverse local flavors. During the Middle Ages, pan-European styles of Romanesque and Gothic cathedrals and abbeys emerged while the Renaissance favored Classical forms implemented by architects known by name. Later, the roles of architects and engineers became separated. Modern architecture began after World War I as an avant-garde movement that sought to develop a completely new style appropriate for a new post-war social and economic order focused on meeting the needs of the middle and working classes. Emphasis was put on modern techniques, materials, and simplified geometric forms, paving the way for high-rise superstructures. Many architects became disillusioned with modernism which they perceived as ahistorical and anti-aesthetic, and postmodern and contemporary architecture developed. Over the years, the field of architectural construction has branched out to include everything from ship design to interior decorating. The notable 19th-century architect of skyscrapers, Louis Sullivan, promoted an overriding precept to architectural design: 'Form follows function'. While the notion that structural and aesthetic considerations should be entirely subject to functionality was met with both popularity and skepticism, it had the effect of introducing the concept of 'function' in place of Vitruvius' 'utility'. 'Function' came to be seen as encompassing all criteria of the use, perception and enjoyment of a building, not only practical but also aesthetic, psychological and cultural. Nunzia Rondanini stated, 'Through its aesthetic dimension architecture goes beyond the functional aspects that it has in common with other human sciences. Through its own particular way of expressing values, architecture can stimulate and influence social life without presuming that, in and of itself, it will promote social development.... To restrict the meaning of (architectural) formalism to art for art's sake is not only reactionary; it can also be a purposeless quest for perfection or originality which degrades form into a mere instrumentality'. Among the philosophies that have influenced modern architects and their approach to building design are Rationalism, Empiricism, Structuralism, Poststructuralism, Deconstruction and Phenomenology. In the late 20th century a new concept was added to those included in the compass of both structure and function, the consideration of sustainability, hence sustainable architecture. To satisfy the contemporary ethos a building should be constructed in a manner which is environmentally friendly in terms of the production of its materials, its impact upon the natural and built environment of its surrounding area and the demands that it makes upon the natural environment for heating, ventilation and cooling, water use, waste products and lighting.