Although Berkeley rejected the Lockean notions of primary and secondary qualities and matter, he retained Locke’s belief in the existence of mind, substance, and causation as an unseen force or power in objects. David Hume, in contrast, rejected all these notions.
David Hume, oil painting by Allan Ramsay, 1766; in the Scottish National Portrait Gallery, Edinburgh.Courtesy of the Scottish National Portrait Gallery
Kinds of perception
Hume recognized two kinds of perception: “impressions” and “ideas.” Impressions are perceptions that the mind experiences with the “most force and violence,” and ideas are the “faint images” of impressions. Hume considered this distinction so obvious that he demurred from explaining it at any length; as he indicated in a summary explication in A Treatise of Human Nature (1739–40), impressions are felt, and ideas are thought. Nevertheless, he conceded that sometimes sleep, fever, or madness can produce ideas that approximate to the force of impressions, and some impressions can approach the weakness of ideas. But such occasions are rare.
The distinction between impressions and ideas is problematic in a way that Hume did not notice. The impression (experience) of anger, for example, has an unmistakable quality and intensity. But the idea of anger is not the same as a “weaker” experience of anger. Thinking of anger no more guarantees being angry than thinking of happiness guarantees being happy. So there seems to be a difference between the impression of anger and the idea of anger that Hume’s theory does not capture.
All perceptions, whether impressions or ideas, can be either simple or complex. Whereas simple perceptions are not subject to further separation or distinction, complex perceptions are. To return to an example mentioned above, the perception of an apple is complex, insofar as it consists of a combination of simple perceptions of a certain shape, colour, texture, and aroma. It is noteworthy that, according to Hume, for every simple impression there is a simple idea that corresponds to it and differs from it only in force and vivacity, and vice versa. Thus, corresponding to the impression of red is the idea of red. This correlation does not hold true in general for complex perceptions. Although there is a correspondence between the complex impression of an apple and the complex idea of an apple, there is no impression that corresponds to the idea of Pegasus or to the idea of a unicorn; these complex ideas do not have a correlate in reality. Similarly, there is no complex idea corresponding to the complex impression of, say, an extensive vista of the city of Rome.
Because the formation of every simple idea is always preceded by the experience of a corresponding simple impression, and because the experience of every simple impression is always followed by the formation of a corresponding simple idea, it follows, according to Hume, that simple impressions are the causes of their corresponding simple ideas.
There are two kinds of impressions: those of sensation and those of reflection. Regarding the former, Hume said little more than that sensation “arises in the soul originally from unknown causes.” Impressions of reflection arise from a complicated series of mental operations. First, one experiences impressions of heat or cold, thirst or hunger, pleasure or pain; second, one forms corresponding ideas of heat or cold, thirst or hunger, pleasure or pain; and third, one’s reflection on these ideas produces impressions of “desire and aversion, hope and fear.”
Because the faculty of imagination can divide and assemble disparate ideas at will, some explanation is needed for the fact that people tend to think in regular and predictable patterns. Hume said that the production of thoughts in the mind is guided by three principles: resemblance, contiguity, and cause and effect. Thus, people who think of one idea are likely to think of another idea that resembles it; their thought is likely to run from red to pink to white or from dog to wolf to coyote. Concerning contiguity, people are inclined to think of things that are next to each other in space and time. Finally and most importantly, people tend to create associations between ideas of things that are causally related. The ideas of fire and smoke, parent and child, and disease and death are connected in the mind for that reason.
Hume used the principle of resemblance for another purpose: to explain the nature of general ideas. He held that there are no abstract ideas, and he affirmed that all ideas are particular. Some of them, however, function as general ideas—i.e., ideas that represent many objects of a certain kind—because they incline the mind to think of other ideas that they resemble. Relations of ideas and matters of fact
According to Hume, the mind is capable of apprehending two kinds of proposition or truth: those expressing “relations of ideas” and those expressing “matters of fact.” The former can be intuited—i.e., seen directly—or deduced from other propositions. That a is identical with a, that b resembles c, and that d is larger than e are examples of propositions that are intuited. The negations of true propositions expressing relations of ideas are contradictory. Because the propositions of arithmetic and algebra are exclusively about relations of ideas, according to Hume, those disciplines are more certain than others. In the Treatise, Hume said that geometry is not quite as certain as arithmetic and algebra, because its original principles derive from sensation, and about sensation there can never be absolute certainty. He revised his views later, however, and, in An Enquiry Concerning Human Understanding (1748), he put geometry on an equal footing with the other mathematical sciences.
Unlike propositions about relations of ideas, propositions about matters of fact are known only through experience. By far the most important of such propositions are those that express or presuppose causal relations—e.g., “Fire causes heat” and “A moving billiard ball communicates its motion to any stationary ball it strikes.” But how is it possible to know through experience that one kind of object or event causes another? What kind of experience would justify such a claim? Cause and effect
In the Treatise, Hume observed that the idea of causation contains three components: contiguity (i.e., near proximity) of time and place, temporal priority of the cause, and a more mysterious component, which he called “necessary connection.” In other words, when one says that x is a cause of y, one means that instances of x and instances of y are always near each other in time and space, that instances of x occur before instances of y, and that there is some connection between x’s and y’s that makes it necessary that an instance of y occurs if an instance of x does.
It is easy to explain the origin in experience of the first two components of the idea of causation. In past experience, all events consisting of a moving billiard ball striking a stationary one were quickly followed by events consisting of the movement of the formerly stationary ball. In addition, the first sort of event always preceded the second and never the reverse. But whence the third component of the idea of causation, whereby one thinks that the striking of the stationary ball somehow necessitates that it will move? That necessity has never been seen or otherwise directly observed in past experience, as have the contiguity and temporal order of the striking and moving of billiard balls.