David Hume (1711-1776) is unquestionably one of the most influential philosophers of the Modern period. Born in Edinburgh, Scotland, his philosophical works include A Treatise on Human Nature (1739), Essays, Moral and Political (2 vols., 1741-1742), An Enquiry Concerning Human Understanding (1748), and An Enquiry Concerning the Principles of Morals (1751). He also published a history of Great Britain and, posthumously, Dialogues Concerning Natural Religion.
Hume belongs to the tradition of British empiricism that includes Francis Bacon (1561-1626), John Locke (1632-1704), and George Berkeley (1685-1753). Common to this tradition is the view that knowledge is founded upon sense-perception, which the human mind passively receives. But whereas Locke and Berkeley believe that human knowledge can go beyond sense-experience, Hume contends in the Introduction of his Treatise that our knowledge is limited to sense-experience, and so offers an empiricism that he argues is more consistent than those of his British predecessors.
Hume’s analysis of the contents of sense-experience begins with the distinction between impressions and ideas. Impressions, which include all our sensations and passions, are more forceful and lively than ideas, which are “the faint images of these in thinking and reasoning” (Treatise, p. 1). Ideas are epistemologically inferior to impressions, and the secondary status that Hume gives them stands in marked contrast to a long tradition in Western philosophy which asserts that universal ideas—not singular sense impressions—are the proper objects of the human intellect. Following Locke, Hume also distinguishes between the simple and complex. Simple impressions and ideas, such as the seeing or imagining of a particular shade of red, admit of no distinction nor separation. Complex impressions and ideas, such as the seeing or imagining of an apple, can be analyzed into their component parts. Whereas all simple ideas are derived from and exactly represent simple impressions, many complex ideas are not, and so their veracity must be called into question. Hume remarks, “When we entertain, therefore, any suspicion that a philosophical term is employed without any meaning or idea (as it but too frequent) we need but enquire, from what impression is that supposed idea derived? And if it be impossible to assign any, this will serve to confirm our suspicion” (first Enquiry, sec. II).
Hume proceeds to show that a number of complex ideas in philosophy, such as the idea of an immaterial self as the core of personal identity, fail to meet his empiricist criterion (see Treatise, Book I, Part IV, sec. VI). But the most famous subject of his criticism is the relation of cause and effect. Western philosophers and scientists traditionally believed that to know something fully one must know the cause upon which it necessarily depends. Hume argues that such knowledge is impossible. He notes that the causal relationship provides the basis for all reasonings concerning matters of fact; however, unlike the relations of ideas explored by mathematics, no judgments that concern matters of fact are necessarily true. This is because we can always imagine, without contradiction, the contrary of every matter of fact (e.g., ‘the sun will not rise tomorrow’ neither is nor implies a contradiction). Hume adds that the causal relationship between any two objects is based on experience, and is not known a priori (e.g., if Adam were created with perfect rational faculties, prior to experience he still could not tell from the properties of water that it would suffocate him.) Yet all that experience establishes concerning causal relationships is that the cause is prior in time to and contiguous with its effect. Experience cannot establish a necessary connection between cause and effect, because we can imagine without contradiction a case where the cause does not produce its usual effect (e.g., we can imagine that a cue ball violently strikes another billiard ball and then, instead of causing the billiard ball to move, the cue ball bounces off it in some random direction). The reason why we mistakenly infer that there is something in the cause that necessarily produces its effect is because our past experiences have habituated us to think in this way. That is, because we have seen in the past that B frequently follows A and never occurs without it, our mind associates B with A such that the presence of one determines the mind to think of the other (see Treatise, Book I, Part III; first Enquiry, sec. IV-V).
Hume maintains that moral distinctions are derived from feelings of pleasure and pain of a special sort, and not—as held by many Western philosophers since Socrates—from reason. Working from the empiricist principle that the mind is essentially passive, Hume argues that reason by itself can never prevent or produce any action or affection. But since morals concerns actions and affections, it cannot be based on reason. Furthermore, reason can influence our conduct in only two ways. First, reason can inform us of the existence of something which is the proper object of a passion, and thereby excite it. Second, reason can deliberate about means to an end that we already desire. But should reason be in error in either of these areas (e.g., by mistaking an unpleasant object for one which is pleasant, or by mistakenly selecting the wrong means to a desired end), it is not a moral but an intellectual failing. As a final point, Hume argues for a distinction between facts and values. According to Hume, one cannot infer conclusions about what ought or ought not to be the case based on premises of what is or is not (see Treatise, Book III, Part I, sec. 1).
Since moral distinctions are not based on reason, Hume infers that they are based on sentiments that are felt by what he calls a “moral sense.” When we describe an action, sentiment, or character as virtuous or vicious, it is because its view causes a pleasure or pain of a particular kind. Hume is well aware that not all pleasures and pains (e.g., the pleasure of drinking good wine) lead to moral judgments. Rather, it is “only when a character is considered in general, without reference to our particular interest, that it causes such a feeling or sentiment, as denominates it morally good or evil” (Treatise, Book III, Part I, sec. 2). Finally, Hume argues that even though moral distinctions are based on feelings, this does not lead to moral relativism. For the general moral principles and the moral sense faculty that recognizes them are common to all human beings (see second Enquiry, “A Dialogue”).
Limitations of space prevent even a cursory sketch of Hume’s treatment of other philosophical questions, such as whether God exists and whether humans have free will and an immortal soul. But the devastating impact of Hume’s empiricism on traditional metaphysics is succinctly summarized by the closing lines of his first Enquiry. “If we take in our hand any volume of divinity or school metaphysics . . . let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.”