He argues that there is no reason to believe in a personal self that continues to exist through time, but only a series of loosely connected impressions that give the illusion of a continuing self because they so closely resemble one another from moment to moment. But in an even more extravagantly sceptical vein, Hume argues that we have no reason whatsoever to expect that things will continue, causally speaking, as they have in the past—that the sun will rise tomorrow; that objects will fall when dropped; or even that the entire world, for all its seeming regularity, will not radically alter or disappear in the next moment. The only “reason” that we have to believe the sun will rise tomorrow, for example, is that it has done so uniformly in the past. But unless one could couple this fact with a “principle of the uniformity of nature”—that things will continue to occur in terms of cause and effect as they have in the past, then past experience can be no guide whatsoever to what will happen or to what we rationally should expect will happen in the future. Yet what proof or support of such a principle of uniformity of nature is there, or could there be, except past experience itself, Hume asks. Past experience is no guide for what to expect in the future unless one accepts the principle of uniformity of nature, and the only thing supporting that principle is past experience itself. Induction, it seems, is viciously circular.

 

Perhaps even more influential than Hume’s scepticism, especially in the twentieth century, is the impact he made on moral philosophy when in book three of the Treatise he calls attention to what he claims is an illicit move from is to ought—from what is the case, to what ought, morally speaking, to be or to be done. He claims that the distinction he draws between is and ought (fact and value) has been surreptitiously and repeatedly ignored in the history of ethics, with the result that the foundations, justification, and nature of ethics has never been properly and adequately accounted for. He says:

In every system of morality . . . the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprized to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. . . . For as this ought, or ought not, expresses some new relation or affirmation, ’tis necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.

Moral judgment, Hume argues, is neither rationally based on, nor intuited from, perceivable facts or objective relations in the natural world. Instead, moral judgments are based on those sentiments that are a function of sympathy which in turn is affected by desire, passion, and innate sensibilities. Passion is necessary to action, and desires and moral sentiments are among the motivating passions. Sentiment usually suffices for moral judgement, but it can be modified by reason on occasion. John Wright, Robert Stecker, and Gary Fuller point out that, according to Hume, “Ethical theory has to take proper account of the role of desire, sentiment or the passions, of innate human tendencies and of the natural and social circumstances in which human beings find themselves.”7 Furthermore, Hume argues that rationalists, in regard to moral judgment, are unable to explain how such judgments can motivate action, whereas his account can. According to Hume, it is moral sentiment rather than reason that produces passions that may cause or prevent certain actions.

We approve of those traits of character—virtues—that make us happy and seem necessary for survival. We disapprove of vices because they bring us unpleasantness and are harmful to existence. The justification of moral sensibility, moral judgment, and ethics in general is not a product of reason but is to be explained in terms of these natural facts—if anywhere. This is not something to be bemoaned on Hume’s account. Indeed, the justification for morality rooted in these natural facts of human nature is far more secure and plays a greater role than any justification based on rational argument—even if the argument were sound—could ever be. Hume’s contentions about morality ran directly counter to just about all received views on morality that preceded him. They continue to be central in moral philosophy.

 

Although he referred to his scepticism as “mitigated,” and some term him a “moderate sceptic,” technically speaking Hume is unable to reject radical scepticism. Instead, he elaborates a theory of reasoning based on experience (a posteriori reasoning) that provides us with a relative sense of justification for believing some of our most common and widely held beliefs—that things will continue, causally speaking, as they have in the past. Thus, Hume did not deny that we should continue to believe much of what we ordinarily believe, in part because we could not do otherwise.