The terms signified and signifier are most commonly related to semiotics, which is defined by Oxford Dictionaries Online as "the study of signs and symbols and their use or interpretation".  Ferdinand de Saussure, a Swiss linguist, was one of the two founders of Semiotics. His book, Course in General Linguistics "is considered to be one of the most influential books published in the twentieth century". Saussure explained that a sign was not only a sound-image but also a concept. Thus he divided the sign into two components: the signifier (or “sound-image”) and the signified (or “concept”). For Saussure, the signified and signifier were purely psychological; they were form rather than substance. Today, the signifier is interpreted as the material form (something which can be seen, heard, touched, smelled or tasted) and the signified as the mental concept.