In this dissertation, I develop an interpretation of Leibniz on modality and free will. I do so for two reasons: first, I am attempting to revitalize the notion that Leibniz is the predecessor of contemporary modal semantics; second, I am using Leibniz's philosophical system to motivate responses to contemporary philosophical issues in modality and free will. In Chapter One, I argue that Leibniz's basic principles are plausible theoretical tools that ought to be used by contemporary philosophers in developing their philosophical (...) systems. In Chapter Two, I develop Leibniz's views on the nature of individuals. I argue that possible individuals are actually of the same sort as individuals in the actual world--possible individuals and actual individuals are complete creatures that do not differ ontologically from each other. In Chapter Three, I argue that Leibniz's views on possible individuals make him a modal realist and compare his view with contemporary modal realism in order to support this claim. I also argue that counterparts avoid many of the problems set for them by contemporary thinkers; I end with the ways that Leibniz's view differs from contemporary accounts. In Chapter Four, I argue that Leibniz provides two different analyses of modality. The first is an infinite analysis account; the second is a possible worlds account. I argue that these two accounts are compatible and amount to two different descriptions of the same theory of modality. I address objections to each account in order to show this. In Chapter Five, I argue that Leibniz is a compatibilist about free will. Importantly, I argue that it is precisely Leibniz's account of modality that allows for this compatibilism, as against a necessitarian like Spinoza. I then use Leibniz's account to challenge contemporary libertarians about free will on the basis of the principle of sufficient reason. I also show how Leibniz can help semicompatibilism avoid a worry concerning necessitarianism. At the end of the day, I claim that adopting elements of Leibniz's system can help us better understand modality and the freedom of the will and can be an aid in furthering contemporary philosophical theory. (shrink)