The source of modal truth

This thesis concerns the source of modal truth. I aim to answer the question: what is it in virtue of which there are truths concerning what must have been the case as a matter of necessity, or could have been the case but isn't. I begin by looking at a dilemma put forward by Simon Blackburn which attempts to show that any realist answer to this question must fail, and I conclude that either horn of his dilemma can be resisted. I then move on to clarify the nature of the propositions whose truth I am aiming to find the source of. I distinguish necessity de re from necessity de dicto, and argue for a counterpart theoretic treatment of necessity de re. As a result, I argue that there is no special problem concerning the source of de re modal facts. The problem is simply to account for what it is in virtue of which there are qualitative ways the world could have been, and qualitative ways it couldn't have been. I look at two ways to answer this question: by appealing to truthmakers in the actual world, or by appealing to non-actual ontology. I develop a theory of truthmakers, but argue that it is unlikely that there are truthmakers for modal truths among the ontology of the actual. I look at the main possibilist ontology, David Lewis' modal realism, but argue that warrant for that ontology is unobtainable, and that we shouldn't admit non-actual possibilia into our ontology. I end by sketching a quasi-conventionalist approach to modality which denies that there are modal facts, but nevertheless allows that we can speak truly when we use modal language.