Joseph Priestley was a man of many and varied intellectual interests. This thesis surveys his philosophical thought, with a central focus on his philosophical theology. The subject can be divided into two parts, natural theology and moral theology. Priestley's natural theology is a perhaps unique attempt to combine and harmonize materialism, determinism and theism, under the auspices of Newtonian methodology. His materialism is based on three arguments: that interaction between matter and spirit is impossible; that a dynamic theory of matter (...) breaks down the active/passive dichotomy assumed by many dualists; and that Newton's "Rules of Reasoning in Philosophy" require the rejection of the "hypothesis" of the soul. His determinism arises from his theory of causation. He attempts to show that the only acceptable account of causation is one in which every cause is invariably followed by the same effect, and that libertarianism violates this central assumption of scientific thought. His theism rests on the Argument from Design. He defends the Argument by trying to show that none of the alternatives to Design advanced in Hume's Dialogues concerning Natural Religion constitute a plausible scientific hypothesis. His critics accused his synthesis of materialism, determinism and theism of leading to atheism or pantheism. His defence against these charges is that his system is essentially theistic, and that its ontology is determined by the Argument from Design. Priestley's moral theology is also in part a response to Hume's Dialogues. He offers a number of replies to the "problem of evil". Evil and adversity heighten our appreciation of good; they are a consequence of the finitude of the creation; they follow from the desirability of having a world governed by general laws; and they make possible the achievement of moral character. Character-formation is his main defence, and his methodology requires him to show that adversity leads more often than not to positive results. This requirement is embodied in his progressivist philosophy of history. History, he holds, exhibits temporal purpose and Design, just as nature exhibits atemporal purpose and Design. Priestley's political thought can be seen as arising from his moral theology, mediated by the notions of "luxury", which he regards as morally harmless, and "idleness", which is the main source of moral evil. His combination of liberalism and radicalism is examined both for internal consistency and as a response to the events of his day, ending with the French Revolution. Priestley is interesting partly for his unorthodoxy, partly for the inter-connectedness of his diverse concerns, and partly because he debated with distinguished contemporaries. Throughout the thesis his views are juxtaposed with those of Hume, Reid, Price, Boscovich, Burke and a number of other thinkers. (shrink)