Introduction
Hume’s introduction presents the idea of placing all science and philosophy on a novel foundation: namely, an empirical investigation into human psychology. He begins by acknowledging “that common prejudice against metaphysical reasonings [i.e., any complicated and difficult argumentation]”, a prejudice formed in reaction to “the present imperfect condition of the sciences” (including the endless scholarly disputes and the inordinate influence of “eloquence” over reason). But since the truth “must lie very deep and abstruse” where “the greatest geniuses” have not found it, careful reasoning is still needed. All sciences, Hume continues, ultimately depend on “the science of man”: knowledge of “the extent and force of human understanding,… the nature of the ideas we employ, and… the operations we perform in our reasonings” is needed to make real intellectual progress. So Hume hopes “to explain the principles of human nature”, thereby “propos[ing] a compleat system of the sciences, built on a foundation almost entirely new, and the only one upon which they can stand with any security”. But an a priori psychology would be hopeless: the science of man must be pursued by the experimental methods of the natural sciences. This means we must rest content with well-confirmed empirical generalisations, forever ignorant of “the ultimate original qualities of human nature”. And in the absence of controlled experiments, we are left to “glean up our experiments in this science from a cautious observation of human life, and take them as they appear in the common course of the world, by men’s behaviour in company, in affairs, and in their pleasures”.
Book 1: Of the Understanding
Part 1: Of ideas, their origin, composition, connexion, abstraction, etc.
Hume begins by arguing that each simple idea is derived from a simple impression so that all our ideas are ultimately derived from experience: thus Hume accepts concept empiricism and rejects the purely intellectual and innate ideas found in rationalist philosophy. Hume’s doctrine draws on two important distinctions: between impressions (the forceful perceptions found in experience, “all our sensations, passions and emotions”) and ideas (the faint perceptions found in “thinking and reasoning”), and between complex perceptions (which can be distinguished into simpler parts) and simple perceptions (which cannot). Our complex ideas, he acknowledges, may not directly correspond to anything in experience (e.g., we can form the complex idea of a heavenly city). But each simple idea (e.g., of the colour red) directly corresponds to a simple impression resembling it—and this regular correspondence suggests that the two are causally connected. Since the simple impressions come before the simple ideas, and since those without functioning senses (e.g., blindness) end up lacking the corresponding ideas, Hume concludes that simple ideas must be derived from simple impressions. Notoriously, Hume considers and dismisses the ‘missing shade of blue’ counterexample.
Briefly examining impressions, Hume then distinguishes between impressions of sensation (found in sense experience) and impressions of reflection (found mainly in emotional experience), only to set aside any further discussion for Book 2’s treatment of the passions. Returning to ideas, Hume finds two key differences between ideas of the memory and ideas of the imagination: the former are more forceful than the latter, and whereas the memory preserves the “order and position” of the original impressions, the imagination is free to separate and rearrange all simple ideas into new complex ideas. But despite this freedom, the imagination still tends to follow general psychological principles as it moves from one idea to another: this is the “association of ideas”. Here Hume finds three “natural relations” guiding the imagination: resemblance, contiguity, and causation. But the imagination remains free to compare ideas along any of seven “philosophical relations”: resemblance, identity, space/time, quantity/number, quality/degree, contrariety, and causation. Hume finishes this discussion of complex ideas with a sceptical account of our ideas of substances and modes: though both are nothing more than collections of simple ideas associated together by the imagination, the idea of a substance also involves attributing either a fabricated “unknown something, in which [the particular qualities] are supposed to inhere” or else some relations of contiguity or causation binding the qualities together and fitting them to receive new qualities should any be discovered.
Hume finishes Part 1 by arguing (following Berkeley) that so-called “abstract ideas” are in fact only particular ideas used in a general way. First, he makes a three-point case against indeterminate ideas of quantity or quality, insisting on the impossibility of differentiating or distinguishing a line’s length from the line itself, the ultimate derivation of all ideas from fully determinate impressions, and the impossibility of indeterminate objects in reality and hence in idea as well. Second, he gives a positive account of how abstract thought actually works: once we are accustomed to use the same term for a number of resembling items, hearing this general term will call up some particular idea and activate the associated custom, which disposes the imagination to call up any resembling particular ideas as needed. Thus the general term “triangle” both calls up an idea of some particular triangle and activates the custom disposing the imagination to call up any other ideas of particular triangles. Finally, Hume uses this account to explain so-called “distinctions of reason” (e.g., distinguishing the motion of a body from the body itself). Though such distinctions are strictly impossible, Hume argues, we achieve the same effect by noting the various points of resemblance between different objects.
Part 2: Of the ideas of space and time
Hume’s “system concerning space and time” features two main doctrines: the finitist doctrine that space and time are not infinitely divisible, and the relationist doctrine that space and time cannot be conceived apart from objects. Hume begins by arguing that, since “the capacity of the mind is limited”, our imagination and senses must eventually reach a minimum: ideas and impressions so minute as to be indivisible. And since nothing can be more minute, our indivisible ideas are “adequate representations of the most minute parts of [spatial] extension”. Upon consideration of these “clear ideas”, Hume presents a few arguments to demonstrate that space and time are not infinitely divisible, but are instead composed of indivisible points. On his account, the idea of space is abstracted from our sense experience (arrangements of coloured or tangible points), and the idea of time from the changing succession of our own perceptions. And this means that space and time cannot be conceived on their own, apart from objects arranged in space or changing across time. Thus we have no idea of absolute space and time, so that vacuums and time without change are ruled out.
Hume then defends his two doctrines against objections. In defending his finitism against mathematical objections, he argues that the definitions of geometry support his account. He then argues that since important geometric ideas (equality, straightness, flatness) do not have any precise and workable standard beyond common observation, corrective measurements, and the “imaginary” standards we are naturally prone to fabricate, it follows that the extremely subtle geometric demonstrations of infinite divisibility cannot be trusted. Next, Hume defends his relationist doctrine, critically examining the alleged idea of a vacuum. No such idea can be derived from our experience of darkness or motion (alone or accompanied by visible or tangible objects), but it is indeed this experience that explains why we mistakenly think we have the idea: according to Hume, we confuse the idea of two distant objects separated by other visible or tangible objects with the very similar idea of two objects separated by an invisible and intangible distance. With this diagnosis in hand, he replies to three objections from the vacuist camp—adding on a sceptical note that his “intention never was to penetrate into the nature of bodies, or explain the secret causes of their operations”, but only “to explain the nature and causes of our perceptions, or impressions and ideas”.
In the final section, Hume accounts for our ideas of existence and of external existence. First, he argues that there is no distinct impression from which to derive the idea of existence. Instead, this idea is nothing more than the idea of any object, so that thinking of something and thinking of it as existent is the very same thing. Next, he argues that we cannot conceive of anything beyond our own perceptions; thus our conception of the existence of external objects is at most a “relative idea”.