Put in the most general terms, Hume’s philosophical project is to investigate the operations of the human mind-employing, inevitably, many of those very operations-so as to shed light on an extraordinarily wide range of important philosophical questions, both in the philosophy of mind and far beyond. In conducting such an investigation, it is essential to understand the entities on which those operations take place and the faculties or powers of the mind by which they take place. The latter are the topic of Chapter Three, while the former constitute the topic of the present chapter. Like many of his predecessors, including Descartes and Locke,

Hume holds: (i) that conscious mental life requires the immediate presence to the mind of certain entities on which mental operations are performed; and (ii) that experience or thought of things that are not immediately present to the mind is accomplished through the capacity of at least some of these entities to represent those other things or to be in some other way of or about them. Whereas his predecessors typically use ‘idea’ as their most general term for these entities, however, Hume employs instead the term ‘perception’ for this purpose, limiting ‘idea’ to a subset of them. At the outset of his investigation in the Treatise, and again in Sec-

tion 2 of the first Enquiry, Hume distinguishes perceptions into “impressions” and (what he calls) “ideas.” He also distinguishes all perceptions-both impressions and ideas-into “simple” and “complex,” and he further distinguishes impressions into “impressions of sensation” and “impressions of reflection.” As he proceeds, he propounds four general principles governing perceptions, each

of which he later invokes in important philosophical arguments. Hume’s four principles of perceptions also facilitate his explanations of several different kinds of ideas. These kinds of ideas include the complex ideas of “substances,” “modes,” and “relations.” They also include “abstract ideas”—that is, concepts-which allow thought to achieve generality so that the mind can think of kinds of qualities and things. He sees his explanation of abstract ideas, inspired in part by Berkeley, as a fundamental contribution to the science of man, and he refers back to it quite often. It is very helpful to keep it in mind when seeking to understand his explanations of mentaland especially conceptual-phenomena. In the dense and neglected but nevertheless important second

part of Treatise Book 1, Hume explains space and time as “manners” in which some perceptions, and the things represented by them, are arranged or (in his terminology) “disposed.” A fundamental question for seventeenth-and eighteenth-century natural philosophers concerned the metaphysics of space and time: Are space and time merely relations among real things, or are they real things in their own right standing in relations of co-location (that is, being in the same place) with other things? A further question was whether finite portions of space and time are infinitely divisibility, a question to which an affirmative answer seemed to many-notably Pierre Bayle (1647-1706)—both mandated by mathematics and yet absurd. Hume’s treatment of space and time is intended to answer both questions and to show in the process how his science of man can provide a new foundation to “all the sciences,” even “Mathematics and Natural Philosophy” [1.2]. Although the Treatise and first Enquiry discuss perceptions as mental

representations at considerable length, neither work addresses in full generality the question of how some-but not all-perceptions are able to represent things at all. Yet Hume’s doctrine, shared with Berkeley, that many perceptions of sight and touch are literally spatially extended provides an important clue, and he says enough in the course of his discussions of various kinds of ideas and impressions to enable us to extract his implicit answer to this crucial question about them. The precise character of Hume’s main distinctions among percep-

tions, the evidential support of his basic principles about them, the

consistency and coherence of his account of their spatial and temporal arrangement, and the content of his views about their representational capacities are all matters of great importance in understanding his philosophy. As is often the case with Hume, however, all have also been matters of some interpretive disagreement. Philosophical novices may wish to survey the discussions of space, time, and mental representation briefly and return to the details later.

Impressions, for Hume, include “all our sensations, passions, and emotions, as they make their first appearance in the soul,” whereas ideas are “the faint images of these in thinking and reasoning” (THN 1.1.1.1/1; see also EHU 2.1-3/17-18). For example, he remarks, “all of the perceptions excited by the present [printed] discourse” are ideas except for those arising immediately from sight and touch, plus any “immediate pleasure or uneasiness” the discourse may produce. This difference between “feeling” (with impressions) and “thinking” (with ideas), he claims, consists essentially in different degrees of what he variously calls “liveliness,” “force,” “vivacity,” “solidity,” “firmness,” and “steadiness.” Some of the many terms that Hume employs interchangeably for

this defining difference between impressions and ideas-for example, ‘liveliness’ and ‘vivacity’—may seem descriptive of an immediately experienced feature or character of perceptions, without any obvious causal implications. Others-such as ‘force’ and ‘steadiness’—seem to many readers more suggestive of causal powers. Hume is very clear, however, that the mind never directly perceives causal power or efficacy as a feature in the causes themselves [4.3, 6.3]; instead, powers can only be recognized through the repeated experience of causes being followed by their effects. Moreover, he seems to take for granted that we can typically discern merely by immediate consciousness or memory whether a perception is an impression or an idea, without the need for any inferences about causal consequences. It seems evident, therefore, that by his ready interchange of a variety of terms he means to capture, in the first instance, a particular immediately experienced feature of perceptions, but one that, as one can soon discover from repeated

experience, also gives those perceptions possessing it greater causal weight and influence in the mind. The mind experiences a somewhat similar feeling, but one having much less causal force and steadiness, in “poetical enthusiasms” (THN 1.3.10.10-12/630-31). In calling his defining difference “vivacity,” Hume sometimes sug-

gests an analogy with visual brightness, but the analogy will seriously mislead if it is mistaken for an identity. In fact, the analogy can mislead in two different ways. First, brightness is a feature only of visual perceptions, whereas all kinds of perceptions are susceptible to differences in mental vivacity on his view. Second, two visual perceptions can exhibit the very same degree of brightness of color while still differing very markedly in their mental vivacity-as, indeed, an impression and an idea of the same bright shade of blue, for example, are bound to do. The point of his analogy is simply that, just as two colors can differ in brightness while still having the same hue (and, as color theorists would say, the same saturation), two perceptions can differ in vivacity while still having all of their other immediately experienced features in common. In order to avoid these sources of confusion and for the sake of both consistency and convenience, I will use Hume’s equally common ‘liveliness’ as the primary term for this immediately experienced feature of some perceptions. When introducing the distinction between impressions and ideas

in the Treatise, Hume remarks that, although these two kinds of perceptions are in general “easily distinguished, it is not impossible in particular instances they may very nearly approach each other.” In sleep, fever, or madness, for example, ideas may “approach to” impressions, whereas some impressions are so “faint and low that we cannot distinguish them from ideas” (THN 1.1.1.1/2; see also EHU 2.1/17, which omits the instance of sleep and the claim that some impressions are faint and low). If Hume grants that some impressions are genuinely equal to some ideas in degree of liveliness, that would of course undermine his previous claim that the difference between the two kinds of perceptions consists in a difference of that very feature. That, in turn, would leave us to look for the basis of the distinction in some other difference, such as a difference of causal origin, accompaniments, or causal consequences. Upon closer inspection, it may be observed that Hume does not

explicitly grant the equality. Ideas could “approach” impressions

without ever quite equaling them, after all, and it may be beyond one’s capacity to “distinguish” in a clear way small differences between fleeting perceptions even if such differences do actually exist. The first Enquiry states that the inability to distinguish impressions from ideas is a result of a mind’s becoming “disordered,” which may suggests an inability to keep track of genuine differences. Nevertheless, as Hume’s initial reference to a “first appearance in

the soul” suggests, impressions and ideas do typically differ in their types of causal origin, in his view. Furthermore, in order to explain how there can be “ideas of ideas,” he remarks, “In thinking of our past thoughts we not only delineate out the objects, of which we were thinking, but also conceive the action of the mind in the meditation, that certain je-ne-scai-quoi, of which ’tis impossible to give any definition or description, but which every one sufficiently understands” (THN 1.3.8.16/106). If this “je-ne-scai-quoi” is unique to ideas (“thinking”), its presence may serve as an indication of the presence of an idea, as contrasted with an impression. Again, impressions and ideas do also differ to some extent in their causal consequences, in his view, and he is clearly interested in the ways in which the causes and effects of similar perceptions tend to differ with degrees of liveliness, even if there turn out to be a few explicable exceptions to these correlations. Because it is at least difficult to see how Hume could be completely confident that fever and madness always involve differences of liveliness too small to be clearly distinguished, it may be reasonable to suggest that he is at least willing to use similarities and differences of causal origin, of accompanying feelings, or of causal consequences to help draw the distinction between impressions and ideas in the relatively rare cases of borderline degrees of felt liveliness. Turning now to a second crucial distinction, Hume declares that

simple perceptions “are such as admit of no distinction nor separation,” while “the complex are the contrary to these, and may be distinguished into parts” (THN 1.1.1.2/2). Locke describes his own distinction between simple and complex ideas in similar terms, and many readers have taken Hume simply to be adopting Locke’s distinction without modification. In fact, however, Hume applies his own simple/complex distinction to perceptions in a

quite different way that often yields quite different results. He offers as an example the complex perception of an apple, which has perceptions of its “colour, taste, and smell” as parts, just as Locke might, but it later emerges that the perception of the color of the whole apple must itself be composed of individual perceptions of the color of spatial parts of the apple [2.4, 2.5]. Similarly, whereas Locke declares the idea of “extension” (that is, spatial extent) to be simple, Hume treats it as complex (“compound”) because it must consist of spatial parts (THN 1.2.3.12-15/38 and THN 1.4.4.8/ 228; see THN 1.1.6.2/16 for evidence of the equivalence of ‘complex’ and ‘compound’). Furthermore, Locke treats some ideas-such as those of existence and unity-as simple and yet inseparable from others, whereas Hume denies that any simple perceptions are inseparable (THN 1.1.7.3/18) and emphasizes that one simple idea can have multiple aspects of resemblance with others and thereby represent many different classes of resembling things (THN 1.1.7.17-18/24-25). The simple/complex distinction plays an especially important role in Hume’s treatments of the Separability Principle [2.2], “distinctions of reason” [2.4], space and time [2.5], and causal “necessary connexions” [6.3]. (See also the discussion of complex abstract ideas and semantic simplicity [4.2-4].) In contrast with the distinctions drawn thus far, the further distinc-

tion that Hume draws between impressions of sensation and impressions of reflection is an entirely causal one. Impressions of sensation “arise from unknown causes,” whereas impressions of reflection “are deriv’d in a great measure from our ideas” (THN 1.1.2.1/7-8). That is, impressions of reflection, unlike impressions of sensation, are feelings that typically arise as reactions to previous ideas, although Hume does not rule out the possibility that on some occasions they may be produced as immediate reactions to other impressions. Impressions of reflection include the various passions-such as love, hatred, pride, humility, joy, grief, hope, fear, desire, and aversion-as well as such other responsive or reactive feelings as “volition” (that is, the feeling of willing), aesthetic pleasure and unease, sentiments of moral approbation and disapprobation, and, in addition, the “impression of necessary connexion” to which he assigns a prominent place in the processes of causal inference and causal judgment.