Hume's writings on ethics began in the 1740 Treatise and were refined in his An Enquiry Concerning the Principles of Morals (1751). He understood feeling, rather than knowing, as that which governs ethical actions, stating that "moral decisions are grounded in moral sentiment." Arguing that reason cannot be behind morality, he wrote:

Morals excite passions, and produce or prevent actions. Reason itself is utterly impotent in this particular. The rules of morality, therefore, are not conclusions of our reason.

Hume's moral sentimentalism was shared by his close friend Adam Smith,[failed verification] and the two were mutually influenced by the moral reflections of their older contemporary, Francis Hutcheson. Peter Singer claims that Hume's argument that morals cannot have a rational basis alone "would have been enough to earn him a place in the history of ethics."

Hume also put forward the is–ought problem, later known as Hume's Law, denying the possibility of logically deriving what ought to be from what is. According to the Treatise (1740), in every system of morality that Hume has read, the author begins by stating facts about the world as it is but always ends up suddenly referring to what ought to be the case. Hume demands that a reason should be given for inferring what ought to be the case, from what is the case. This is because it "seems altogether inconceivable, how this new relation can be a deduction from others."

Hume's theory of ethics has been influential in modern-day meta-ethical theory, helping to inspire emotivism, and ethical expressivism and non-cognitivism,[failed verification] as well as Allan Gibbard's general theory of moral judgment and judgments of rationality.

In his discussion of miracles, Hume argues that we should not believe miracles have occurred and that they do not therefore provide us with any reason to think God exists. In An Enquiry Concerning Human Understanding (Section 10), Hume defines a miracle as "a transgression of a law of nature by a particular volition of the Deity, or by the interposition of some invisible agent". Hume says we believe an event that has frequently occurred is likely to occur again, but we also take into account those instances where the event did not occur:

A wise man ... considers which side is supported by the greater number of experiments. ... A hundred instances or experiments on one side, and fifty on another, afford a doubtful expectation of any event; though a hundred uniform experiments, with only one that is contradictory, reasonably beget a pretty strong degree of assurance. In all cases, we must balance the opposite experiments ... and deduct the smaller number from the greater, in order to know the exact force of the superior evidence.

Hume discusses the testimony of those who report miracles. He wrote that testimony might be doubted even from some great authority in case the facts themselves are not credible: "[T]he evidence, resulting from the testimony, admits of a diminution, greater or less, in proportion as the fact is more or less unusual."

Although Hume leaves open the possibility for miracles to occur and be reported, he offers various arguments against this ever having happened in history. He points out that people often lie, and they have good reasons to lie about miracles occurring either because they believe they are doing so for the benefit of their religion or because of the fame that results. Furthermore, people by nature enjoy relating miracles they have heard without caring for their veracity and thus miracles are easily transmitted even when false. Also, Hume notes that miracles seem to occur mostly in "ignorant and barbarous nations" and times, and the reason they do not occur in the civilised societies is such societies are not awed by what they know to be natural events. Hume recognizes that over a long period of time, various coincidences can provide the appearance of intention. Finally, the miracles of each religion argue against all other religions and their miracles, and so even if a proportion of all reported miracles across the world fit Hume's requirement for belief, the miracles of each religion make the other less likely.

Hume was extremely pleased with his argument against miracles in his Enquiry. He states, "I flatter myself, that I have discovered an argument of a like nature, which, if just, will, with the wise and learned, be an everlasting check to all kinds of superstitious delusion, and consequently, will be useful as long as the world endures." Thus, Hume's argument against miracles had a more abstract basis founded upon the scrutiny, not just primarily of miracles, but of all forms of belief systems. It is a commonsense notion of veracity based upon epistemological evidence, and founded on a principle of rationality, proportionality and reasonability.

The criterion for assessing Hume's belief system is based on the balance of probability whether something is more likely than not to have occurred. Since the weight of empirical experience contradicts the notion for the existence of miracles, such accounts should be treated with scepticism. Further, the myriad of accounts of miracles contradict one another, as some people who receive miracles will aim to prove the authority of Jesus, whereas others will aim to prove the authority of Muhammad or some other religious prophet or deity. These various differing accounts weaken the overall evidential power of miracles.[failed verification]

Despite all this, Hume observes that belief in miracles is popular, and that "the gazing populace… receive greedily, without examination, whatever soothes superstition, and promotes wonder."

Critics have argued that Hume's position assumes the character of miracles and natural laws prior to any specific examination of miracle claims, thus it amounts to a subtle form of begging the question. To assume that testimony is a homogeneous reference group seems unwise- to compare private miracles with public miracles, unintellectual observers with intellectual observers and those who have little to gain and much to lose with those with much to gain and little to lose is not convincing to many. Indeed, many have argued that miracles not only do not contradict the laws of nature but require the laws of nature to be intelligible as miraculous, and thus subverting the law of nature. For example, William Adams remarks that "there must be an ordinary course of nature before anything can be extraordinary. There must be a stream before anything can be interrupted." They have also noted that it requires an appeal to inductive inference, as none have observed every part of nature nor examined every possible miracle claim, for instance those in the future. This, in Hume's philosophy, was especially problematic.

Little appreciated is the voluminous literature either foreshadowing Hume, in the likes of Thomas Sherlock or directly responding to and engaging with Hume—from William Paley, William Adams, John Douglas, John Leland, and George Campbell, among others. Regarding the latter, it is rumoured that, having read Campbell's Dissertation, Hume remarked that "the Scotch theologue had beaten him."

Hume's main argument concerning miracles is that miracles by definition are singular events that differ from the established laws of nature. Such natural laws are codified as a result of past experiences. Therefore, a miracle is a violation of all prior experience and thus incapable on this basis of reasonable belief. However, the probability that something has occurred in contradiction of all past experience should always be judged to be less than the probability that either one's senses have deceived one, or the person recounting the miraculous occurrence is lying or mistaken, Hume would say, all of which he had past experience of. For Hume, this refusal to grant credence does not guarantee correctness. He offers the example of an Indian Prince, who, having grown up in a hot country, refuses to believe that water has frozen. By Hume's lights, this refusal is not wrong and the prince "reasoned justly;" it is presumably only when he has had extensive experience of the freezing of water that he has warrant to believe that the event could occur.

So, for Hume, either the miraculous event will become a recurrent event or else it will never be rational to believe it occurred. The connection to religious belief is left unexplained throughout, except for the close of his discussion where Hume notes the reliance of Christianity upon testimony of miraculous occurrences. He makes an ironic remark that anyone who "is moved by faith to assent" to revealed testimony "is conscious of a continued miracle in his own person, which subverts all principles of his understanding, and gives him a determination to believe what is most contrary to custom and experience." Hume writes that "All the testimony whichever was really given for any miracle, or ever will be given, is a subject of derision."