Bodies of knowledge are, with a few exceptions, not designed to be taught, but to be used. To teach a body of knowledge is thus a highly artificial enterprise. thus a highly artificial enterprise. The transition from knowledge regarded as a tool to be put to use, to knowledge as something to be taught and learnt, is precisely what I have termed the didactic transposition of knowledge.

Chevallard, Y. (1988, August). On didactic transposition theory: Some introductory notes. In International Symposium on Research and Development in Mathematics, Bratislava, Czechoslavakia.