 The term phoneme was first used by the French philologist Dufres Degenit, who coined the term for an individual sound as an element of a sound inventory. At the end of the 19th century, Nikolai Krushevsky, a member of the famous Kazan school, regarded the phoneme as a phonetically indivisible unit. In 1916, the term was taken up by Ferdinand de Saussure, who defined the term as a concrete speech sound produced by the articulatory systems in real time. Today, the phoneme is defined as an abstract phonological unit that serves as a reference model for a set of speech sounds related to each other. There are three ways in which to consider the phoneme today. The physical or phonetic view of the phoneme goes back to Daniel Jones, who defined the phoneme as a head term of a family of speech sounds that satisfy two criteria, phonetic similarity and complementary distribution. The definition of the phoneme in purely phonological or functional terms is characteristic of the Prague School of Linguistics, where Nikolai Trubetskoy, one of the leading scholars of this school, interpreted the phoneme as a minimal unit that can function to distinguish meaning. The psychological view, as developed by the Polish linguist Baudelande Coutonet, defines the phoneme as a mental reality, as the intention of the speaker or the impression of the hearer, or both. Despite some theoretical problems with the phoneme as the one and only phonological unit, it is still used as a major unit in phonology today.