There is also a parallel with the three interlocking systems of language distinguished by Halliday (1978), consisting the forms, meaning and functions of language. In the sociology of mathematics, Restivo (1985) distinguishes the syntactical and semantic properties of an object (following Hofstadter), paralleling the syntaxsemantic distinction. Hersh (1988) makes an analogous distinction between the ‘front’ and ‘back’ of mathematics. Restivo (1988) also distinguishes between ‘social’ and ‘technical’ talk of mathematics, paralleling the distinction between the third level of pragmatic considerations and the first two levels taken as one, respectively. Thus precursors of these three levels, in various forms, are to be found in the literature.