At least three major theistic arguments have a common structure, in which their proponents i) defend the existence of some ultimate, abstract entity and then ii) claim that this abstract thing they have argued for is God. One of these arguments is called the “Third Way.” The conclusion of the Third Way is that a) there exists a being that is itself necessary and that is also the source (or cause) of all other metaphysically necessary beings, and b) this ultimate, necessary being is God. Aquinas’s other arguments, i.e. the First, Second, Fourth, and Fifth Ways, are structured similarly. The abstract entity whose existence is established in a) is not shown to have any personal properties at all. The entity seems strikingly unlike the personal deity whose existence the argument is intended to support. So there is an objection according to which the concept of an ultimate source of necessity is so removed from that of a personal God that the argument fails to support its conclusion. In this dissertation I show that, as a result of some relatively recent developments in modal logic and some associated insights into the nature of meaning and reference, this objection no longer holds.In the first chapter, I argue for the existence of propositions. Propositions are the objects of a person’s beliefs, attitudes, and the conceptual “vehicles” of truth and falsehood; something is true or false if and only if the proposition corresponding to that very thing is true or false. To argue for the existence of propositions, I argue that sentences which seem to refer to propositions cannot be analyzed as other sentences that refer to arguably less abstract entities, such as language types, inscriptions, or other concrete instances of language production. This part of the dissertation involves an analysis of the nature of translation, especially with respect to the translation of direct quotation, as well as some discussion of the semantics of the English word ‘said’. In the second chapter, I then defend what I call a “dirty propositional” account of modal-logic semantics. I do this by arguing that, although worlds are not identical to sets or classes of propositions, for each possible world there are characteristic sets of propositions that correspond to that world. I also argue that any one of a world’s characteristic sets entails every proposition that is true at that world. Appreciating that worlds have characteristic sets allows for an analysis of modal statements that is committed to a minimalistic metaphysics of possible worlds, is not committed to a counterpart theory, and allows us to use sets of propositions to refer to worlds. In the third chapter, I use the semantics for modal logic developed in the second chapter to discuss the medieval, Aristotelian concepts of necessity and possibility that Aquinas uses in the Third Way. In this discussion I show that even though the modal concepts in the argument are not the same as contemporary modal concepts, results from twentieth-century modal logic and philosophy of language are nonetheless relevant to the argument. They show that the so-called “Impersonal Objection” or “Gap Problem” does not apply to the Third Way. More generally, this dissertation shows that the strength of similar objections to Aquinas’s arguments from the concept of a Prime Mover or a First Cause also depends at least partly on the semantic properties of the descriptions that are used to express the concept of a Prime Mover and the concept of a First Cause.