I trace the principles known as the indiscernibility of identicals, "Leibniz's Law", and the principle of substitutivity, beginning with Aristotle, through Leibniz, Frege, and Russell, and culminating in Quine. I argue that the indiscernibility of identicals is an ontological principle and the principle of substitutivity is a linguistic principle. I discuss the relations and conflations of the principles and various attempts to defend the principle of substitutivity from apparent counter-examples, focussing on Quine's attempt to use the principle as a criterion (...) for referential and non-referential occurrences of singular terms. I find Quine's acount ambivalent and counter-intuitive, yet I think his insight that pronouns or variables are paradigmatic of reference is significant. With that as a cue, I suggest an analysis of all singular terms into a counting device and an identifying component. I argue, finally, that the principle of substitutivity holds fundamentally for the counting element, and only "by courtesy" for unanalyzed singular terms. (shrink)