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The basic goal of the first three of these works is indicated by the subtitle of the ''Treatise''; "an attempt to introduce the experimental method into moral subjects". [[CiteRef::Morris and Brown (2016)|p.7]] Hume sought to extend Newton's experimental philosophy from natural philosophy into what was then called '''moral philosophy''', which he defined as the "science of human nature". [[CiteRef::Morris and Brown (2016)|p.8]] The field of moral philosophy was much broader then than today, and included topics that we might classify as psychology or cognitive science, as well as epistemology. To Hume, an understanding of the workings of the mind was the key to establishing the foundations of all other knowledge, including "Mathematics, Natural Philosophy, and Natural Religion". [[CiteRef:: Norton (2009)|p. 34]] Natural philosophers, like Newton and Boyle, he maintains, had cured themselves of their "passion for hypotheses and systems". [[CiteRef:: Morris and Brown (2016)|p. 8-9]] Hume sought to work the same cure for moral philosophy, which he saw as full of speculative metaphysical hypotheses and constant dispute. [[CiteRef::Morris and Brown (2016)]] He proposed an empiricist alternative to ''a priori'' metaphysics based on pure reason and its the speculative belief systemsto which it led. [[CiteRef::Norton (2009)]] He was a naturalist who rejected any appeal to the supernatural in explanations of human nature. For such beliefs, and because he argued that we cannot justify many of our beliefs, he is noted as a skeptic. But he also observed that we have non-rational faculties which compel certain sorts of beliefs (such as the belief that there is a world external to my mind of which my senses provide knowledge), and it is these faculties of which he wishes to give a positive descriptive account. [[CiteRef::Biro (2009)]]

In a letter to Henry Home (1696-1782) published in 1737, Hume confessed that he intended to include a skeptical discussion of miracles in his ''Treatise'' but left it out for fear of offending readers. Critics of religion in eighteenth century Europe faced the risk of fine, imprisonment, or worse. [[CiteRef::Fieser (2016)]] Hume did later publish his critique in the ''Enquiry'' in 1748. He wrote that "A wise man...proportions his belief to the evidence" [[CiteRef::Hume (2008)| p. 56]] and drew the conclusion that "A miracle is a violation of the laws of nature; and because firm and unalterable experience has established these laws, the case against a miracle is- just because it is a miracle- as complete as any argument from experience can possibly be imagined to be....No testimony is sufficient to establish a miracle unless it is of such a kind that its falsehood would be more miraculous than the fact it tries to establish...When anyone tells me that he saw a dead man restored to life, I immediately ask myself whether it is more probable that this person either deceives or has been deceived or that what he reports really has happened...If the falsehood of his testimony would be more miraculous than the event he relates, then he can claim to command my belief or opinion, but not otherwise". [[CiteRef:: Hume (2008)| p. 58-59]] The claim that a dead man was restored to life is, of course, central to Christian theology. Hume's arguments have relevance beyond theological knowledge, and have been cited in relation to other sorts of extraordinary or surprising claims, such as claims of paranormal occurrences or extraterrestrial intelligence. They are succinctly summarized by the maxim, popularized by the twentieth century astronomer Carl Sagan (1934-1996), that "extraordinary claims demand extraordinary evidence". [[CiteRef::Sagan (1979)| p. 62]][[CiteRef:: Deming (2016)]] In 1757, Hume published an essay entitled ''The Natural History of Religion'' which was the first systematic attempt to explain religious belief solely in terms of what we would call psychological and sociological factors. [[CiteRef::Fieser (2016)]]