1. Barcan, R. (1947). The identity of individuals in a strict functional calculus of second order. Journal of Symbolic Logic, 12, 12–15.
2. Cartwright, R. (1971). Identity and substitutivity. In M. K. Munitz (Ed.), Identity and Individuation (pp. 119–133). New York: New York University Press.
3. Dummett, M. (1973). Frege: Philosophy of language. London: Duckworth.
4. Geach, P. T. (1963). Quantification theory and the problem of identifying objects of reference. Acta Philosophica Fennica, 16, 41–52.
5. Haack, S. (1978). Philosophy of logics. Cambridge: Cambridge University Press.