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Design theory refers to understanding the methods, strategies, research and analysis of the practice of "design.
The essay "Ornament and Crime" by "Adolf Loos from 1908 is one of the early 'principles' design-theoretical texts. Others include Le Corbusier's Vers une architecture, and "Victor Papanek's Design for the real world (1972).
In a 'principles' approach to design theory, the "De Stijl movement promoted a geometrical abstract, "ascetic" form of purism that was limited to functionality. This modernist attitude underpinned the "Bauhaus movement. Principles were drawn up for design that were applicable to all areas of modern aesthetics.
For an introduction to the philosophy of design see the article by Per Galle at the Royal Danish Academy.
An example of early design science was Altshuller's Theory of inventive problem solving, known as "TRIZ, from Russia in the 1940s. Herbert Simon's 1969 The sciences of the artificial began a more scientific basis to the study of design.  Since then the further development of fields such as "design methods, "design research, "design science and "design thinking has promoted a wider understanding of design theory.
Design theory is linked to many other disciplines, including the humanities, engineering, social sciences, history and philosophy.
In the art historical discourse, the question of the distinction between art and design has been discussed repeatedly. The design theory of "Walter Gropius can be seen as a continuation of art theory, whereas this relationship is called into question in the environment of the HFG Ulm under "Otl Aicher, "Gui Bonsiepe and "Tomás Maldonado.
In the field of "engineering and "medicine "ergonomics and "human computer interface are to key areas where design theory plays an important role. Within the field of "economics, "marketing affects the parameters of the design. In the 1930s, "Henry Dreyfuss developed the field of ergonomic design through his application of "anthropometrics.
In the field of "sociology, "actor-network theory developed by, among others, "Bruno Latour, has proven to be influential. Actor-network theory describes things and objects as active and examines how they direct human action.
In the field of "anthropology, "Yana Milev developed an approach where design anthropology provides an epistemology, phenomenology and survey of the varieties of the extended concept of design. Here the design concept is placed at the centre of the nexus of meaning of cultural production that rests on the three pillars Segno, Mythus and "Techne. Anthropological design research is trans-disciplinary, developing in the connexion between Visual Culture (signal, in/visibility, image/void, imagination, representation), Doing Culture (act, cooperation, relation, fabrication, exchange), Material Culture (object, artefact, thing, facing, texture), Knowledge Culture (techniques, practices, norms, beliefs, values), Narrative Culture (mythology, significance, meaning, memory, identity), Critical Culture (watching, criterion, antagonism, crisis, theory) and Aesthetic Culture (emotion, sentiment, taste, feel, sense).