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We present a new solution to long-standing puzzles about substitution of co-referential terms. Our solution is based on a notion of perspective, where the speaker’s perspective can be differentiated from the perspective of the agent whose thoughts, beliefs, etc., the speaker is reporting. Our formalization is a conservative extension of the simply-typed lambda calculus utilizing monads, a construction in category theory that provides a way to map a set of objects and functions into a more complex space of objects and functions. We offer a lexicalist analysis of perspective whereby certain lexical items introduce potential shifts of perspective while others do not. We show that this provides the means to give a general semantics of perspective with respect to substitutability, allowing us to capture not just the standard embedded cases of non-substitutability of distinct but co-referential terms, but also cases involving no embedding and no distinct terms. We also show that our semantics generalizes to cases outside the nominal domain, such as synonymous natural kind terms and other predicates.