[1] This distinction has been discussed, in general terms, by Freudenthal (Freudenthal, H.: 1988, `Logic as a subject or as an attitude`, in Barra, M. and Zanardo, A. (eds), Atti degli Incontri in Logica Matematica, XII Incontro: La Logica Matematica nella didattica, Padova).

[2] Association of Symbolic Logic, 1995, Guidelines for Logic Education, Bulletin of Symbolic Logic, Vol. 1, n. 1, 4-7)