One of the main arguments Russell offers for the Theory of Descriptions in “On Denoting” is that it can be used to solve a puzzle about substitutivity. This article argues that, owing to a widespread mischaracterization of the substitutivity principle Russell means to be defending, neither the puzzle itself nor Russell's solution to it have been well understood. The principle Russell seeks to defend concerns not the substitution of expressions in a sentence but rather the substitution of propositional constituents in a Russellian proposition. This article further argues that Russell's solution requires him to adopt certain substantive views about the nature of the referents of what are usually called “logically proper names.” In particular, Russell's solution will work only if the referents of such names are given to one who understands them in a manner that is entirely “aspect-free.” Russell's notion of acquaintance, since it fits this bill, is therefore motivated by his solution to the puzzle, as is his choice of sense-data to be the referents of genuine Russellian names of particulars. Finally, the article argues that since a version of the George IV puzzle arises for propositions, Russell's solution commits him to regarding some instances of sentences as incomplete symbols and to denying that we have acquaintance with propositions. This in turn leads Russell in Principia Mathematica to adopt the Multiple Relation Theory of Judgment, and thus to embrace agnosticism about propositions. His further move to the stronger position of full eliminativism concerning propositions is motivated only by his late-in-the-day espousal of the Principle of Sufficient Reason.

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