Abstract: This paper upholds Fitch's suggestion that it is reasonable to suppose that names of identified objects should be everywhere intersubstitutable when they are being used as names (the principle of substitutivity). Counterexamples provided by R. Cartwright are diagnosed as trading on the inevitable ambiguities of natural languages, and it is argued that the principle of substitutivity should therefore be tested with respect to statements in logical form.

Keywords: Cartwright, definite descriptions, Fitch, logical form, principle of substitutivity, proper names, substitutivity,