David Hume's Influences

1. Hume's Influences
In a 1737 letter, Hume wrote that readers of the Treatise would benefit by looking at writings by Nicolas Malebranche, George Berkeley, Pierre Bayle, and René Descartes:

I shall submit all my Performances to your Examination, & to make you enter into them more easily, I desire of you, if you have Leizure, to read once over le Recherche de la Verité of Pere Malebranche, the Principles of Human Knowledge by Dr Berkeley, some of the more metaphysical Articles of Baile's Dictionary; such as those of Zeno, & Spinoza. Des-Cartes Meditations wou'd also be useful, but I don't know if you will find it easily among your Acquaintances. These Books will make you easily comprehend the metaphysical Parts of my Reasoning. And as
to the rest, they have so little Dependence on on [sic] all former Systems of Philosophy, that your natural Good Sense will afford you Light enough to judge of their Force & Solidity. [Hume to Michael Ramsay, August 26, 1737]
Chronologically, the first philosopher on Hume's list is René Descartes (1596–1650). In his Meditations on the First Philosophy (1641), Descartes combats sceptics who doubt the existence of the external world and the reliability of our senses. To accomplish his task, Descartes himself provisionally plays the role of a sceptic and doubts everything that can possibly be doubted. Descartes then arrives at one absolute truth – his own existence – and uses this as a foundation for demonstrating all knowledge. Hume was probably influenced by Descartes's provisional doubting process, as Hume himself doubted the sources of human knowledge. Throughout Hume's philosophical writings, though, he also reacted against the more speculative metaphysical views that Descartes developed.
The remaining three philosophers listed in Hume's letter – Malebranche, Bayle, and Berkeley – were controversial figures when their writings first appeared, and they share the conviction that the true nature of the world is not as evident as we ordinarily think.
French Catholic philosopher Nicolas Malebranche (1638–1715) was a follower of Descartes and is most remembered for his Search After Truth (1674–1675). Two themes stand out in that work, both of which influenced Hume. First, Malebranche wrestled with how our minds receive perceptual images from external objects. For example, as I stand in front of a tree, I have a visual image of that tree. How does the tree itself cause that image in my mind? For Malebranche, the tree is physical in nature, yet my perceptual image is spiritual in nature, and, so, something like a miracle must take place to convert the one to the other. After rejecting various theories of perception, Malebranche concludes that God possesses mental/spiritual images of all external things, and that he implants these ideas in our minds at the appropriate time – when I stand before the tree, for example. In short, according to Malebranche, we see external objects by viewing their images as they reside in God. Hume did not adopt Malebranche's theological solution to this problem, but perhaps Hume learned from Malebranche that there is a great gulf between external objects and our perceptions of them, and that it is exceedingly difficult to explain the
connection between the two.

The second major theme in Malebranche concerns the nature of causality, or, more specifically, how two events (such as the motion of a stick that strikes and moves a ball) are causally connected. Malebranche argues that physical objects by themselves simply cannot be the cause of motion in other objects; only spirits can do that. So, when a stick strikes a ball, some spiritual force must intervene and actually cause the ball to move. Malebranche concludes that God is the true cause of the ball's motion, and that the movement of the stick is only the occasion, the occasional cause, of the ball's motion. Malebranche pushes this theory further and argues that God is also the true cause behind human bodily motion. For example, when I wilfully pick up a book, my will is only the occasional cause, and God is the true cause. Again, Hume did not adopt Malebranche's theological solution to the problem of causality, but it was perhaps through Malebranche that Hume became aware of the difficulty of explaining the nature of causal connections.

Although Malebranche raised questions about our knowledge of external objects and causality, he was nevertheless optimistic about the ability of our human reason to unravel these philosophical mysteries. However, influenced by the ancient Greek sceptical traditions, French philosopher Pierre Bayle (1647–1706) was much more pessimistic about our rational abilities. A philosophy and history professor, Bayle made a lasting mark in philosophy with his monumental Historical and Critical Dictionary (Dictionaire historique et critique, 1692). The Dictionary contains hundreds of articles on notable figures from ancient through modern times, and in lengthy footnotes to these articles Bayle presents his own original and often radical views. In the letter cited above, Hume mentions "the more metaphysical Articles of Baile's Dictionary" and cites two particular articles: Zeno of Elea and Spinoza.

Zeno (. c. 450 BCE) was a follower of Greek philosopher Parmenides and, like his teacher, Zeno argued that our ordinary notions of the world are illusions. Zeno presents a series of logical paradoxes that show the inherently contradictory nature of motion and space. Bayle comments extensively on Zeno's paradoxes and suggests that space is composed of one of three possible things: mathematical points, indivisible physical points, or infinitely divisible parts. Bayle sceptically concludes that all three of these views are absurd, and, so, no adequate explanation of space is possible. Drawing on Bayle's discussion, Hume concludes with an almost equally sceptical assessment regarding our notions of space and time. Benedict de Spinoza (1632–1677) – another controversial modern philosopher – argued that God is the single substance of the entire universe. What appear to be individual objects, such as rocks and trees, are in fact only modifications of God's single-substance. Bayle treats Spinoza contemptuously and argues that it is counterintuitive to see all physical things as modifications of a single substance. In the Treatise Hume extends Bayle's critique further and argues that theologians are equally counterintuitive when they say, for example, that my diverse mental images are really unified in my single spirit-mind.

A third metaphysical article in Bayle that certainly had an impact on Hume is that on Pyrrho (c. 365–c. 275 BCE). Pyrrho was the founder of one of the Greek sceptical traditions, which survives principally in the writings of Sextus Empiricus (. c. 200 CE). In his article on Pyrrho, Bayle discusses the Pyrrhonian assault on both perceptual knowledge and knowledge of self-evident truths. Bayle largely agrees with Pyrrho and argues further that human reason collapses under the weight of its own inherent paradoxes. Ultimately, for Bayle, we must reject reason as a guide for truth and rely instead on religious faith. It is probably from Bayle that Hume learned to use faith as a shield to protect him from accusations of atheism or any other negative consequence of sceptical philosophy. In the same article Bayle discusses the common philosophical distinction between what Locke later dubbed primary and secondary qualities; Bayle argues that they are both ultimately spectator-dependent. This is a line of reasoning that Hume also offers.

The last philosopher that Hume mentions in the letter is Anglican Bishop George Berkeley (1685–1753), published two of his key works while in his tewnties, namely A Treatise Concerning the Principles of Human Knowledge (1710) and Three Dialogues between Hylas and Philonous (1713). In both of these works Berkeley argues against the existence of an external material world. For Berkeley, our experience of external reality is nothing more than a continuing stream of perceptions, nor can we say anything intelligible about any physical substance that supposedly causes these perceptions. Berkeley concludes that we must reject the theory of physical reality and instead recognise that God directly feeds us perceptions of external things. Although rejecting Berkeley's theological solution, Hume adopts Berkeley's arguments showing our inability to access some external world behind our perceptions. Berkeley also critically discusses the view that there is no reality to our individual minds beyond the stream of perceptions that we experience. Berkeley rejects this view and instead argues that individual minds do exist; however, Hume seems to advance a similar problem, while denying Berkeley's solution.

In addition to the writings of Descartes, Malebranche, Bayle, and Berkeley, there were undoubtedly other philosophers that directly influenced Hume's metaphysical views. An avid admirer of the Roman philosopher Cicero (106–43 BCE), Hume was familiar with Cicero's Academica, a dialogue on the nature and possibility of acquiring knowledge. Perhaps most importantly, Hume was influenced by An Essay concerning Human Understanding (1690) of John Locke (1632–1704). In this work Locke argues that the root of all knowledge lies in experience; Hume shares this view with Locke.

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2. Summary of Treatise Book 1
In his short autobiography, "My Own Life," Hume notes that he composed his Treatise in his mid-twenties while on retreat for three years in France. It is a long and complex book that systematically re-thinks a wide range of philosophical issues. The first two books of the Treatise appeared simultaneously in 1739. We will briefly look at some of the major themes in both books.

Book 1, titled "Of the Understanding," opens analysing various categories of mental events, which roughly follow this scheme:

Perceptions
A. Ideas
1. From memory
2. From imagination
a. From fancy
b. From understanding
(1) Involving relations of ideas
(2) Involving matters of fact
B. Impressions
1. Of sensation (external)
2. Of reflection (internal)
He first divides all mental perceptions between ideas (thoughts) and impressions (sensations and feelings), and then makes two central claims about the relation between ideas and impressions. First, adopting what is commonly called Hume's copy thesis, he argues that all ideas are ultimately copied from impressions. That is, for any idea we select, we can trace the component parts of that idea to some external sensation or internal feeling. This claim places Hume squarely in the empiricist tradition, and throughout Book 1 he uses this principle as a test for determining the content of an idea under consideration. Second, adopting what we may call Hume's liveliness thesis, he argues that ideas and impressions differ only in terms of liveliness. For example, my impression of a tree is simply more vivid than my idea of that tree. His early critics pointed out an important implication of the liveliness thesis, which Hume himself presumably hides. Most modern philosophers held that ideas reside in our spiritual minds, whereas impressions originate in our physical bodies. So, when Hume blurs the distinction between ideas and impressions, he is ultimately denying the spiritual nature of ideas and instead grounding them in our physical nature. In short, these critics argue that, for Hume, all of our mental operations – including our most rational ideas – are physical in nature.

Hume next notes that there are several mental faculties that are responsible for producing our various ideas. He initially divides ideas between those produced by the memory, and those produced by the imagination. The memory is a faculty that conjures up ideas based on experiences as they happened. For example, the memory I have of my drive to the store is a comparatively accurate copy of my previous sense impressions of that experience. The imagination, by contrast, is a faculty that breaks apart and combines ideas, thus forming new ones. He uses the familiar example of a golden mountain: this idea is a combination of an idea of gold and an idea of a mountain. As our imagination chops up and forms new ideas, it is directed by three principles of association, namely, resemblance, contiguity, and cause and effect. For example, by virtue of resemblance, the sketch of a person leads me to an idea of that actual person. The ideas of the imagination are further divided between two categories. Some imaginative ideas represent flights of the fancy, such as the idea of a golden mountain; other imaginative ideas, though, represent solid reasoning, such as predicting the trajectory of a thrown ball. The fanciful ideas are derived from the faculty of the fancy, and are the source of fantasies, superstitions, and bad philosophy. By contrast, the good ideas are derived from the faculty of the understanding – or reason – and roughly involve either mathematical demonstration or factual predictions. Hume notes that, when we imaginatively exercise our understanding, our minds are guided by seven philosophical or "reasoning" relations, which are divided as follows:

Principles of reasoning concerning relations of ideas (yielding demonstration): (1) resemblance, (2) contrariety, (3) degrees in quality, and (4) proportions in quantity or number

Principles of reasoning concerning matters of fact (yielding judgments of probability): (5) identity, (6) relations in time and place, and (7) causation

Armed with the above conceptual distinctions, he turns his attention to an array of standard philosophical problems. As he examines them one by one, he repeatedly does three things. First, he sceptically argues that we are unable to gain complete knowledge of some important philosophical notion under consideration. Second, he shows more positively how the understanding gives us a very limited idea of the notion under consideration. Third, he explains how some erroneous views of that notion are grounded in the fancy, and he accordingly recommends that we reject those ideas. For convenience, we will follow this three-part scheme as we consider Hume's discussions.
Space. (1) On the topic of space, Hume argues that we have no ideas of infinitely divisible space (1.2.2.2). (2) When accounting for the idea we do have of space, he argues that "the idea of space is convey'd to the mind by two senses, the sight and touch; nor does any thing ever appear extended, that is not either visible or tangible" (1.2.3.15). Further, he argues that these objects – which are either visible or tangible – are composed of finite atoms or corpuscles, which are themselves "endow'd with colour and solidity." These impressions are then "comprehended" or conceived by the imagination; it is from the structuring of these impressions that we obtain our idea of space. (3) In contrast to this idea of space, Hume argues that we frequently presume to have an idea of space that lacks visibility or solidity. He accounts for this erroneous notion in terms of a mistaken association that people naturally make between visual and tactile space (1.2.5.21).

Time. (1) Hume's treatment of our idea of time is like his treatment of the idea of space. He first maintains that we have no idea of infinitely divisible time (1.2.4.1). (2) He then notes Locke's point that our minds operate at a range of speeds that are 'd by the original nature and constitution of the mind, and beyond which no influence of external objects on the senses is ever able to hasten or retard our thought" (1.2.3.7). The idea of time, then, is not a simple idea derived from a simple impression; instead, it is a copy of impressions as they are perceived by the mind at its fixed speed (1.2.3.10). (3) In contrast to this account of time, he argues that we frequently entertain a faulty notion of time that does not involve change or succession. The psychological account of this erroneous view is that we mistake time for the cause of succession instead of seeing it as the effect (1.2.5.29).

Necessary connection between causes and effects. (1) Hume sceptically argues that we cannot get an idea of necessary connection through abstraction or by observing it through sensory experiences (1.3.14.12 ff.). (2) The idea we have of necessary connection arises as follows: we experience a constant conjunction of events A and B – that is, repeated sense experiences where events resembling A are always followed by events resembling B. This produces a habit such that upon any further appearance of A, we expect B to follow. This, in turn, produces an internal feeling "to pass from an object to the idea of its usual attendant," which is the impression from which the idea of necessary connection is copied (1.3.14.20). (3) A common but mistaken notion on this topic is that necessity resides within the objects themselves. He explains this mistaken belief by the natural tendency we have to impute subjectively perceived qualities to objects (1.3.14.24).

External objects. (1) Hume's sceptical claim here is that we have no conception of the existence of external objects (1.2.6.9). (2) Never-theless, he argues, we do have an unavoidable "vulgar" or common belief in the continued existence of objects, and this idea he accounts for. His explanation is lengthy, but involves the following features. Perceptions of objects are disjointed and have no unity of themselves (1.4.2.29). In an effort to organize our perceptions, we first naturally assume that there is no distinction between our perceptions and the objects that are perceived (this is the so-called "vulgar" view of perception). We then conflate all ideas (of perceptions), which put our minds in similar dispositions (1.4.2.33); that is, we associate resembling ideas and attribute identity to their causes. Consequently, we naturally feign the continued and external existence of the objects (or perceptions) that produced these ideas (1.4.2.35). Lastly, we go on to believe in the existence of these objects because of the force of the resemblance between ideas (1.4.2.36). Although this belief is philosophically unjustified, Hume feels he has given an accurate account of how we inevitably arrive at the idea of external existence. (3) In contrast to the previous explanation of this idea, he recommends that we doubt a more sophisticated but erroneous notion of existence – the so-called philosophical view – which distinguishes between perceptions and external objects that cause perceptions. The psychological motivation for accepting this view is this: our imagination tells us that resembling perceptions have a continued existence, yet our reflection tells us that they are interrupted. Appealing to both forces, we ascribe interruption to perceptions and continuance to objects (1.4.2.52).

Personal identity. (1) Hume's sceptical claim on this issue is that we have no experience of a simple, individual impression that we can call the self (1.4.6.2) – where the "self" is the totality of a person's conscious life. (2) Nevertheless, we do have an idea of personal identity that must be accounted for. He begins his explanation of this idea by noting that our perceptions are fleeting, and he concludes from this that all we are is a bundle of different perceptions (1.4.6.4). Because of the associative principles, though, the resemblance or causal connection within the chain of our perceptions gives rise to an idea of oneself, and memory extends this idea past our immediate perceptions (1.4.6.18 ff.). (3) A common abuse of the notion of personal identity occurs when the idea of a soul or unchanging substance is added to give us a stronger or more unified concept of the self (1.4.6.6).

In all of these discussions, Hume performs an interesting balancing act between making sceptical attacks (step 1) and offering positive theories (step 2). In the conclusion to Book 1, though, he appears to elevate his scepticism to a higher level and exposes the inherent contradictions in even his best philosophical theories. He notes three such contradictions. One centres around what we call induction. Our judgments based on past experience all contain elements of doubt; we are then impelled to make a judgment about that doubt, and – since this too is based on past experience – this in turn will produce a new doubt. Once again, though, we are impelled to make a judgment about this second doubt, and the cycle continues. He concludes that "no finite object can subsist under a decrease repeated in infinitum." A second contradiction involves a conflict between two theories of external perception – our natural inclination to direct realism vs. the copy theory of perception of philosophers. The third contradiction involves a conflict between causal reasoning and belief in the continued existence of matter. After listing these contradictions, Hume despairs over the failure of his metaphysical reasoning:

The intense view of these manifold contradictions and imperfections in human reason has so wrought upon me, and heated my brain, that I am ready to reject all belief and reasoning, and can look upon no opinion even as more probable or likely than another. [1.4.7.8]
He then subdues his despair by recognizing that nature forces him to set aside his philosophical speculations and return to the normal activities of common life. He recognizes, though, that in time he will be drawn back into philosophical speculation in order to attack superstition and educate the world.

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3. Summary of the Treatise Book 2
Book 2 of the Treatise is a study of impressions of reflection, in contrast with impressions of sensation. Locke had discussed ideas of reflection as "being such only as the mind gets by reflecting on its own operations within itself" (Essay 2.2.4). For Locke, these are introspective experiences of our mental faculties such as remembering, willing, discerning, reasoning, and judging. Immediately parting company with Locke, reflective impressions for Hume are passions – that is, emotions – and not introspective experiences of our mental faculties. Book 2 is largely a study of the various passions.

Hume opens Book 2 offering a taxonomy of types of passions, which we may outline here:

Reflective Impressions
1. Calm (reflective pleasures and pains)
2. Violent
a. Direct (desire, aversion, joy, grief, hope, fear)
b. Indirect (love, hate, pride, humility)
Hume initially divides passions between the calm and the violent. He concedes that this distinction is somewhat fuzzy, but he explains that people commonly distinguish between types of passions in terms of their degrees of forcefulness. Adding more precision to this common distinction, for Hume calm passions are emotional feelings of pleasure and pain associated with moral and aesthetic judgments. For example, according to Hume, when I see a person commit a horrible deed, I will experience a feeling of pain. When I view a good work of art, I will experience a feeling of pleasure.
In contrast to the calm passions, violent ones constitute the bulk of our emotions, and violent passions divide between direct and indirect passions.
For Hume, "direct passions" are so called because they arise immediately – without complex reflection on our part – whenever we see something good or bad. For example, if I consider an unpleasant thing, such as being burglarised, then I will feel the passion of aversion. The key direct passions are desire, aversion, joy, grief, hope, and fear. He suggests that sometimes these passions are sparked instinctively – as by, for example, my desire for food when I am hungry. Others, though, are not connected with instinct and are more the result of social conditioning. There is an interesting logic to the six direct passions, which Hume borrowed from a tradition that can be traced to ancient Greek Stoicism. We can diagram the relation between the six with this
chart:

When good/bad objects are considered abstractly desire (towards good objects) aversion (towards evil objects)
When good/bad objects are actually present joy (towards good objects) grief (towards evil objects)
When good/bad objects are only anticipated hope (towards good objects) fear (towards evil objects)
Compare, for example, the passions that I will experience regarding winning the lottery vs. having my house burglarized. Suppose that I consider them purely in the abstract – or "consider'd simply" as Hume says (2.3.9.6). I will then desire to win the lottery and have an aversion towards being burglarized. Suppose that both situations are actually before me; I will then experience joy over winning the lottery and sorrow over being burglarized. Suppose, finally, that I know that at some unknown time in the future I will win the lottery and be burglarized. I will then experience hope regarding the lottery and fear of being burglarized.
Hume devotes most of Book 2 to an analysis of the indirect passions, and this analysis is his unique contribution to theories of the passions. The four principal passions are love, hate, pride, and humility. These passions are called "indirect" since they are the secondary effects of a previous feeling of pleasure and pain. Suppose, for example, that I paint a picture, which gives me a feeling of pleasure. Since I am the artist, I will then experience an additional feeling of pride. Hume explains in great detail the psychological process that triggers indirect passions such as pride. Specifically, he notes that these passions arise from a double relation between ideas and impressions, which we can illustrate here with the passion of pride:

(1) I have an initial idea of some possession (or "subject"), such as my painting, and this idea gives me pleasure.
(2) Through the associative principle of resemblance, I then immediately associate this feeling of pleasure with a resembling feeling of pride; this association constitutes the first relation in the double relation.
(3) This feeling of pride then causes me to have an idea of myself (as the "object" of pride).
(4) Through some associative principle such as causality, I then associate the idea of myself with the idea of my painting (which is the "subject" of my pride); this association constitutes the second relation in the double relation.
According to Hume, the three other principal indirect passions arise in parallel ways. For example, if my painting is ugly and causes me pain, then I will experience the secondary passion of "humility" – perhaps more accurately expressed as humiliation. By contrast, if someone else paints a pleasing picture, then this will trigger in me a feeling of "love" for that artist – perhaps more accurately expressed as esteem, which is another term that Hume uses. If the artist paints a painfully ugly picture, then this will trigger in me a feeling of "hatred" towards the artist – perhaps more accurately expressed as disesteem.
The most lasting contribution of Book 2 of the Treatise is Hume's argument that human actions must be prompted by passion, and never can be motivated by reason. Thus, Hume concludes that "Reason is, and ought only to be, the slave of the passions" (2.3.3.4). Looking more closely at the motivations behind our actions, he the issue of liberty and necessity, and he comes down strongly on the side of necessity. Hume's discussion here follows the three-step scheme that he used in Book 1. (1) He rejects the notion of liberty that denies necessity and causes (2.3.1.18). (2) He then argues that all mental or physical actions produced by the will arise from antecedent motives, tempers, and circumstances (2.3.1.5 ff.). Making use of his definitions of causality, he argues that these motives produce actions (mental or physical) that have the same causal necessity that we observe in external objects. (3) Lastly, he explains why people commonly believe in an uncaused will (2.3.2.1 ff.). Among other causes of this mistaken belief is the fact that people erroneously believe that they have an experience of liberty owing to a mistaken association: first, people have an idea of liberty (or lack of determination); next, when performing actions they experience a "looseness" which resembles their idea of liberty. Summary of the Enquiry, and the "Dissertation on the Passions." In "My Own Life," Hume states his opinion that the Treatise failed largely because of its style, rather than its content:

I had always entertained a notion, that my want of success in publishing the Treatise of Human Nature, had proceeded more from the manner than the matter, and that I had been guilty of a very usual indiscretion, in going to the press too early. I, therefore, cast the first part of that work anew in the Enquiry concerning Human Under-standing, which was published while I was at Turin.
Accordingly, Hume reworked some of the contents of Books 1 and 2 into his Enquiry Concerning Human Understanding – although he included much additional material that does not appear in the Treatise. Hume's Enquiry was first published anonymously in 1748 under the title Philosophical Essays Concerning Human Understanding. The title page ascribes the work to "the author of the Essays moral and political;" Hume's authorship, though, was no secret since the 1748 edition of the Essays includes his name. We do not know precisely when Hume began this book, although we know from a correspondence that he was working on it in 1745. Prior to publication, he circulated a manuscript of the book among his friends for comments. A close friend Henry Home, later Lord Kames (1696–1782), tried to talk Hume out of publishing the work because of its sceptical content and the controversy that it would provoke. Hume ignored the advice and in a letter to Home wrote that he did not care about the consequences:
The other work [soon to be published] is the Philosophical Essays [i.e., the Enquiry], which you dissuaded me from printing. I won't justify the prudence of this step, any other way than by expressing my indifference about all the consequences that may follow. [Hume to Henry Home, February 9, 1748]
The style of the Enquiry is in fact quite different than that of the Treatise. It is much shorter, more informal, and does not aim to present a comprehensive theory of human nature. Its original title – Philosophical Essays Concerning Human Understanding – reflects its place within the 18th century genre of essay writing. That is, it is a collection of twelve loosely related philosophical essays. The underlying theme that ties the twelve essays together is the importance of experience and causal inference in establishing our ideas. Briefly, these are the central themes of the Enquiry's twelve sections.
1. Of the Different Species of Philosophy: Hume describes two styles of philosophical writing: an easy-reading philosophy grounded in common life, and a difficult-reading philosophy grounded in abstract concepts. He explains the value of both and proposes to mix elements of the two styles in his Enquiry.

2. Of the Origin of Ideas: Hume argues that ideas differ from impressions only by being less lively, and that all ideas are copied from impressions. He concisely states his test for meaning: to see if "a philosophical term is employed without any meaning ... we need but enquire, from what impression is that supposed idea derived?"

3. Of the Association of Ideas: Hume argues that the only three principles of association of ideas are resemblance, contiguity, and cause and effect. Unlike in the Treatise, which describes these as principles of the imagination, here Hume states more generally that they apply in the operations of both the memory and imagination. All editions of the Enquiry except that of 1777 – containing Hume's final revisions – include a lengthy discussion of the use of associative principles in epic poetry writing and history writing.

4. Sceptical Doubts concerning the Operations of the Understanding:

Hume notes that the objects of the faculty of understanding (or reason) are either relations of ideas or matters of fact. He devotes this section to uncovering the foundations of our reasoning concerning matters of fact. Such reasoning is based on cause and effect relations, which in turn are based on experience, without the aid of reason or our imagination (that is, the fancy). This in turn raises the question of how we make inductive generalizations in experience.

5. Sceptical Solution of these Doubts: Hume goes on to argue that inductive generalizations in experience result from the principle of "custom or habit." He next examines how belief arises. For Hume, belief is a more vivid conception of an object than we would otherwise have through the imagination (that is, the fancy) alone. The ideas in which we believe become more "intense and steady" through habit and custom. He concludes showing how the principles of association can intensify an idea and thus produce belief.

6. Of Probability: Hume explains the difference between chances and probability. Chances involve situations in which there are at least two possible outcomes, each of which may occur equally. Probability, on the other hand, entails that we have experienced one event to occur more frequently than another. He then shows how belief arises with both chance and probability.

7. Of the Idea of Necessary Connexion: Hume explains the origin of our idea of causal power using his copy thesis. He first argues that necessary connection does not arise from an outward sense impression. Neither does it arise from an internal impression – from, that is, a "reflection on the operations of our own minds;" (Hume here follows Locke's notion of reflective impressions rather than the notion found in the Treatise). Specifically, it does not arise from reflecting on willed bodily motions encountering a resistive physical force, the willed creation of thoughts, or the experience of God as the true cause (as the Occasionalists claim). Ultimately, the idea of causal power is based on the "customary transition of the imagination from one object to its usual [i.e., constantly conjoined] attendant." He concludes by offering two definitions of causality based on his notion of causal power.

8. Of Liberty and Necessity: Hume defends the necessitarian point of view by arguing that all human actions are caused by antecedent motives. He offers several illustrations of the connection between motives and actions that fit his two definitions of causality. He reconciles necessity with liberty by defining liberty as "a power of acting or not acting, according to the determinations of the will" – which is similar to Locke's definition. Hume notes the criticism that necessity undermines morality since it eliminates moral choice. In response he argues that we rely on necessity to link a person's actions with his motives and thus pass moral judgment on that person's actions. He also notes the criticism that necessity forces us to trace all evil human actions back through a causal chain to God. He suggests possible solutions to this problem, but concludes that it is a mystery that human reason is not fit to handle.

9. Of the Reason of Animals: Hume argues that what he has said about cause and effect, induction, habit and belief is confirmed by observing the same processes in animals. In a footnote he lists nine points that distinguish degrees of human intelligence from animal intelligence and that also distinguish the degrees of the reasoning ability of intelligent humans and not-so-intelligent humans.

10. Of Miracles: Hume argues that empirical judgments – including those based on testimony – involve weighing evidence for and against a given claim. According to Hume, the empirical testimony of uniform laws of nature will always outweigh the testimony of any alleged miracle. Hume notes four factors that count against the credibility of most miracle testimonies: the witnesses lack integrity; we have a propensity to sensationalize; miracle testimonies abound in barbarous nations; and miracles support rival religious systems. However, he continues, even the most credible miracle testimonies (which presumably are not decisively weakened by these four factors) are still outweighed by the evidence of consistent laws of nature. Although people typically see miracles as the foundation of their religion, Hume argues that this is unreasonable. He suggests that Christianity in particular is better founded on faith, rather than on miracle testimony. Christianity indeed requires belief in miracles, but such belief should involve an act of faith and not reason.

11. Of a Particular Providence and of a Future State: Originally titled "Of the Practical Consequences of Natural Religion," this section presents a fictional conversation in which two characters examine some of the traditional philosophical arguments about the nature and existence of God. The sceptical character principally attacks the design argument and the argument that God rewards or punishes human actions either in this life or the next.

12. Of the Academical or Sceptical Philosophy: Hume describes different kinds of scepticism, defending some types and rejecting others. He associates Pyrrhonian scepticism with blanket attacks on all reasoning about the external world, abstract reasoning about space and time, or causal reasoning about matters of fact. He argues, though, that we must reject such scepticism since "no durable good can ever result from it." Instead, Hume recommends a more moderate or Academic scepticism that tempers Pyrrhonism by, first, exercising caution and modesty, and, second, restricting our speculations to abstract reasoning and matters of fact.

In 1757 Hume published a work titled Four Dissertations, the second item in which was titled "Of the Passions." Hume later incorporated this piece into his Essays and Treatises, and, paralleling the arrangement of the three Books of the Treatise, he placed it between the Enquiry Concerning Human Understanding and the Enquiry Concerning the Principles of Morals. This brief work – titled "Dissertation on the Passions" – is an abbreviated version of much of Book 2 of the Treatise, and many parts of it are taken word for word from that earlier work.

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5. Overview of Early Responses
When Books 1 and 2 of the Treatise appeared in 1739, little immediate interest was shown in it. Hume reflects on this unfortunate fact in "My Own Life": "Never literary attempt was more unfortunate than my Treatise of Human Nature. It fell dead-born from the press, without reaching such distinction, as even to excite a murmur among the zealots." Indeed, it did not generate a flurry of critical responses by pamphleteers or offended clergymen. However, within a year of publication, four reviews of the Treatise did in fact appear in scholarly review journals. All of the reviews were restricted to the contents of Book 1 of the Treatise, with no discussion of Hume's theory of the passions from Book 2. Only one of these – in The History of the Works of the Learned – was in English, and this was executed by a reviewer who himself admits that he was not philosophically up to the task of grasping a work as complex as the Treatise. The reviewer was severely critical and, among his comments, he argued that the causal proofs for God's existence are "utterly demolished" by Hume's rejection of the principle that "whatever begins to exist, must have a cause of existence." The reviewer's point is a recurring theme among Hume's early critics, and even today some philosophers discuss the extent to which the causal proofs for God's existence are affected by Hume's notion of causality. The short review from the German Göttingische Zeitungen was also critical. The French review journal Bibliothèque raisonnée published a generally positive review heavily dependent on Hume's own Abstract of the Treatise. The French review journal Nouvelle bibliothèque published a neutral review that consisted mainly of a translation of passages from the Treatise.

Aside from review journals, the first early response to Hume's metaphysical views was a brief article, in 1740, in Common Sense: or the Englishman's Journal. The anonymous author criticized Hume's view of necessity for its dangerous implication that our behaviour is beyond our control. In 1745 Hume's sceptical and antireligious views in the Treatise came under fire when Hume became a candidate for the chair of moral philosophy at the University of Edinburgh. A list of charges drawn up by William Wishart was circulated. These were incorporated into Hume's response and published as A Letter from a Gentleman by Henry Home in 1745.

The Enquiry first appeared in 1748 and, in "My Own Life," Hume notes that, like the Treatise, the Enquiry did not at first draw critical attention:

But this piece was at first little more successful than the Treatise of Human Nature. On my return from Italy, I had the mortification to find all England in a ferment, on account of Dr. Middleton's Free Enquiry, while my performance was entirely overlooked and neglected.
Not only were there no immediate critical responses to the Enquiry, but the work does not appear to have even been reviewed in any British periodical. The absence of such reviews is not surprising since there were no scholarly review journals in Great Britain at the time, and more popular periodicals only sporadically included reviews. Within two years, though, critical responses to "Of Miracles" appeared, and these soon brought notoriety to the Enquiry as a whole. Although the Enquiry was not reviewed in Great Britain, it was in fact reviewed twice in the German Göttingische Zeitungen. The first of these reviews, which appeared in 1749, was favourable, but a review appearing in 1753 was mixed, with especially critical comments on "Of Miracles."
In 1751 Hume wrote in a letter that he rejected the Treatise as an immature work:

I believe the philosophical Essays [i.e., the Enquiry] contain every thing of Consequence relating to the Understanding, which you woud meet with in the Treatise; & I give you my Advice against reading the latter. By shortening & simplifying the Questions, I really render them much more complete. Addo dum minuo. The philosophical Principles are the same in both: But I was carry'd away by the Heat of Youth & Invention to publish too precipitately. So vast an Undertaking, plan'd before I was one and twenty, & compos'd before twenty five, must necessarily be very defective. I have repented my Haste a hundred, & a hundred times. [Hume to Gilbert Eliot, March or April 1751]
Contrary to Hume's wishes, critical discussions of the Treatise continued to appear with more frequency. One of these was in Essays on the Principles of Morality and Natural Religion (1751) by Henry Home. Although respecting Hume's philosophical abilities, in this work Home critically discusses Hume's theory of belief and personal identity.
Around this time Hume became one of the secretaries of the Philosophical Society of Edinburgh, first founded in 1731. He held this post probably until 1763 and during that time was coeditor with Alexander Monro of two volumes of Essays and Observations that were read at the Society's meetings. The first volume appeared in 1754 and opened with an essay by Henry Home titled "Of the Laws of Motion" (pp. 1–69). The second item in the collection is a critical and somewhat abusive discussion of Home's essay by John Stewart (d. 1766) titled "Some Remarks on the Laws of Motion, and the Inertia of Matter." In this essay, Stewart includes a brief paragraph criticizing Hume's view of causality and personal identity:

That something may begin to exist, or start into being without a cause, hath indeed been advanced in a very ingenious and profound system of the sceptical philosophy; but hath not yet been adopted by any of the societies for the improvement of knowledge. Such sublime conceptions are far above the reach of an ordinary genius; and could not have entered into the head of the greatest physiologist on earth. The man who believes that a perception may subsist without a percipient mind or a perceiver, may well comprehend, that an action may be performed without an agent, or a thing produced without any Cause of the production. And the author of this new and wonderful doctrine informs the world, that, when he looked into his own mind, he could discover nothing but a series of fleeting perceptions; and that from thence he concluded, that he himself was nothing but a bundle of such perceptions. [Pages 70–140]
A note to this paragraph states, "Treatise on Human Nature, 3 vols. octavo. This is the system at large, a work suited only to the comprehension of Adepts. An excellent compend or sum whereof, for the benefit of vulgar capacities, we of this nation enjoy in the Philosophical Essays, and the Essays Moral and Political."Prior to its publication, Hume read Stewart's essay and was bothered by Stewart's contemptuous tone towards both Home and Hume himself. In a letter to Stewart, Hume suggests – and probably bluffs – that as editor of the volume Hume could have equally abused Stewart in the Preface to the work. However, Hume states "I am so great a Lover of Peace, that I am resolv'd to drop this Matter altogether, & not to insert a Syllable in the Preface, which can have a Reference to your Essay." Hume continues in the letter objecting on philosophical grounds to Stewart's distortion of Hume's actual views:
But allow me to tell you, that I never asserted so absurd a Proposition as that any thing might arise without a Cause: I only maintain'd, that our Certainty of the Falshood of that Proposition proceeded neither from Intuition nor Demonstration; but from another Source. That Caesar existed, that there is such an Island as Sicily; for these Propositions, I affirm, we have no demonstrative nor intuitive Proof. Woud you infer that I deny their Truth, or even their Certainty? There are many different kinds of Certainty; and some of them as satisfactory to the Mind, tho perhaps not so regular, as the demonstrative kind.
Where a man of Sense mistakes my Meaning, I own I am angry: But it is only at myself: For having exprest my Meaning so ill as to have given Occasion to the Mistake.

In a tone similar to his letter to Elliot above, Hume next tells Stewart that he regrets publishing his Treatise at all:
That you may see I wou'd no way scruple of owning my Mistakes in Argument, I shall acknowledge (what is infinitely more material) a very great Mistake in Conduct, viz my publishing at all the Treatise of human Nature, a Book, which pretended to innovate in all the sublimest Parts of Philosophy, & which I compos'd before I was five & twenty. Above all, the positive Air, which prevails in that Book, & which may be imputed to the Ardor of Youth, so much displeases me, that I have not Patience to review it. [Hume to John Stewart, c. February 1754]
Two years after the conflict with Stewart, the Philosophical Society published their second volume of Essays and Observations, which included an essay by Thomas Melvill (1726–1753) titled "Observations on Light and Colours," which includes a criticism of Hume's view of the indivisibility of extension. Around the same time John Leland criticized Hume's view of causality in his A View of the Principal Deistical Writers (1755–1756).
In 1757 Hume's Four Dissertations appeared; this included his "Dissertation on the Passions." Although none of the earlier reviews of the Treatise discussed Book 2, three reviews of Four Dissertations discuss "Of the Passions." Two of the reviews are not very enthusiastic. The Literary Magazine states "The second essay is on the passions, in which, as in the former case, we do not perceive any thing new. This we should not mention if we were not talking of an author fond of novelty." The Critical Review similarly concludes its discussion by saying, "This whole dissertation, to say the truth, appears to us very trite and superficial; and unworthy of so eminent a writer. But no authors are always equal to themselves." William Rose's review in the Monthly Review states more positively that what Hume "says upon the subject, is extremely ingenious, and deserves the philosophical reader's attentive perusal."

The publication of Thomas Reid's An Inquiry into the Human Mind (1764) marks a turning point in early discussions of Hume's metaphysics. Although no less critical of Hume than earlier respondents, Reid nevertheless had deep respect for Hume's philosophical abilities and saw him as "the greatest Metaphysician of the Age." According to Reid, Hume's ruthlessly sceptical philosophy is the logical outcome of a philosophical stance that began with Descartes, and which Reid calls the "theory of ideas." According to this theory, we do not perceive external things directly, but instead we only experience perceptual images – or "ideas" – of external things. The sceptical consequence of this is that we must question the existence of everything except these perceptual images

– including external objects and even the human mind, which allegedlyhouses these perceptions. And, according to Reid, this is what Hume did. As Reid himself became an important philosophical figure throughout Europe and America, many writers perpetuated his interpretation of Hume. We find, for example, a condensed statement of Reid's view in the following by George L. Scott:

Locke had admitted matter, spirit, and ideas. By many passages, one would be apt to thing that he saw no absurdity in material Spirit, or in spiritual Matter. Berkeley comes, sees the difficulty, and strikes out matter. Then comes a Paresian Egoist, who strikes out all spirit, but his own. And, lastly, our friend Hume, strikes out even his own spirit, and leaves nothing but Ideas! [George L. Scott to Lord Monboddo, April 3, 1773]
Aside from Reid, in the final two decades of Hume's life, a variety of other philosophers wrote in reaction to his metaphysical views. Richard Price, in his Review of the Principal Questions and Difficulties in Morals (1758) criticizes Hume's discussion of induction in the Treatise. Joseph Highmore published a brief essay against Hume's view of necessity in his Essays, Moral, Religious, and Miscellaneous (1766). Scottish philosophers were particularly interested in responding to Hume. James Balfour's Philosophical Essays (1768) criticizes Hume's view of academic scepticism and necessary connection. In his Appeal to Common Sense (1766–1772), James Oswald attacks a variety of sceptical and anti-religious themes within Hume's writings. The most prominent critic of this period was James Beattie who devoted a large portion of his Essay on the Nature and Immutability of Truth (1770) to refuting many of Hume's philosophical views. In the first volume of his Origin and Progress of Language (1773) James Burnett, Lord Monboddo criticized Hume's distinction between ideas and impressions.
Although Hume's Enquiry was the most common target of attack by these philosophers, some also pointed their guns at offending portions of the Treatise. Near the close of his life in 1775, Hume composed an advertisement to the second half of his collected philosophical works, Essays and Treatises on Several Subjects, in which he officially denounced his Treatise, and expressed his wish to be remembered on the basis of his Essays and Treatises:

Most of the principles, and reasonings, contained in this volume, were published in a work in three volumes, called A Treatise of Human Nature: A work which the Author had projected before he left College, and which he wrote and published not long after. But not finding it successful, he was sensible of his error in going to the press too early, and he cast the whole anew in the following pieces, where some negligences in his former reasoning and more in the expression, are, he hopes, corrected. Yet several writers who have honoured the Author's Philosophy with answers, have taken care to direct all their batteries against that juvenile work, which the author never acknowledged, and have affected to triumph in any advantages, which, they imagined, they had obtained over it: A practice very contrary to all rules of candour and fair-dealing, and a strong instance of those polemical artifices which a bigotted zeal thinks itself authorised to employ. Henceforth, the Author desires, that the following Pieces may alone be regarded as containing his philosophical sentiments and principles.
Hume offers here an inaccurate chronology as to when he projected the Treatise. Hume "left college" at around age 14, and, according to his letter to Elliot, he began his work on the Treatise at age 21. The change in chronology is apparently in effort to distance himself from the Treatise into an increasingly remote past. Hume here refers to "several writers" who attacked the views found in the Treatise. Of the early critics listed so far, Reid and Beattie come the closest to matching Hume's description of writers who have directed "all their batteries against that juvenile work." In spite of Hume's public disavowal, philosophers continued to challenge the views of the Treatise. In the first two volumes of his Ancient Metaphysics (1779, 1782), Monboddo continued his attack on Hume. Reid similarly developed his criticisms of Hume in his two great works, Essays on the Intellectual Powers of Man (1785) and Essays on the Active Powers of Man (1788). Joseph Priestley, who throughout his voluminous writings regularly comments on Hume, had mixed views of Hume's metaphysics. In his Letters to a Philosophical Unbeliever (1780), Priestley attacked the Enquiry section by section, hoping to put Hume's unjustified fame in proper perspective. On the other hand, in his Doctrine of Philosophical Necessity Illustrated (1777), Priestley largely endorsed Hume's view of necessity and in the preface to that work recommends to readers "some things very well written on it by Mr. Hume, and Lord Kaims." Priestley's defence of necessity was so successful that it overshadowed Hume's view in the free will and determinism debate in the late 18th century. An exception to this, though, was James Gregory's Philosophical and Literary Essays (1792), which, in a 300 page introductory essay, attacks Hume's account of necessity. In his Illustrations of Mr. Hume's Essay Concerning Liberty and Necessity (1795), John Allen defends Hume against Gregory.
As the 19th century approached, philosophers narrowed their interest in Hume's metaphysics largely to his notion of causality. We see this in George Gleig's article on "Metaphysics" in the third edition of the Encyclopædia Britannica (1797), Henry James Richter's article "Hume's View of Necessary Connection" in the Monthly Magazine (1797), and Richard Kirwan's Remarks (1801). Two events around this time drew further interest to Hume's view of causality. The first is Immanuel Kant's Critique of Pure Reason (1781), which by 1800 was gaining notice in Great Britain. In the Critique and later in the Prolegomena (1783), Kant describes his metaphysical system as an attempt to answer the problem that Hume raised about causality. This sparked immediate interest in Hume's theory within Germany. Although it was some time before Kant's writings were translated into English, a few primers on Kant appeared in English and these drew attention to Kant's intellectual debt to Hume. One of these was A.F.M. Willich's Elements of the Critical Philosophy (1798), which translates Kant's discussion of Hume in the Prolegomena.

The second event surrounding interest in Hume's theory of causality was political in nature. In 1805, Scottish scientist John Leslie was a candidate for the chair of Mathematics at the University of Edinburgh. Several local clergy who opposed Leslie's appointment exploited the fact that he endorsed Hume's view of causality, a view which they believed undermined the causal proof for God's existence. Two prominent Scottish philosophers came to Leslie's rescue and published works defending Hume's view of causality. Dugald Stewart published a pamphlet titled A Short Statement of Some Important Facts, Relative to the Late Election of a Mathematical Professor in the Univ. of Edinburgh (1805). In this Stewart lists respected scholars who adopted Hume's view of causality and notes that "I found that the passage [on causality] objected to contained nothing… but what I myself, and many others much wiser and better than me, had openly avowed as their opinions." Thomas Brown also published several pamphlets during the controversy, and compiled a two-volume collection titled Tracts, Historical and Philosophical... Respecting the Election of Mr. Leslie to the Professorship of Mathematics (1806). A year later, Brown greatly expanded one of his pamphlets as Observations on the Nature and Tendency of the Doctrine of Mr. Hume (1806). Brown's work is an insightful and sophisticated early discussion of Hume's view of causality, of great merit even by contemporary standards. Brown's Humean view of causality was adopted by physician William Lawrence in his Lectures on physiology (1817).

Amidst the dominant focus on his view of causality, occasional discussions of other topics in Hume appeared. Thomas Cogan, in his Treatise on the Passions (1807), criticized different aspects of Hume's theory of the passions. Cogan also published a section-by-section critical commentary of Hume's Enquiry in his Ethical Questions (1817). Dugald Stewart discussed Hume's account of why we venerate the past in Philosophical Essays (1810) and Hume's scepticism in Dissertation on the Progress of Philosophy (1821). In Biographia Literaria (1817), Samuel Taylor Coleridge drew attention to the similarities between the views of Aquinas and Hume on the association of ideas, and thereby sparked a discussion in Blackwood's Magazine (1818). Thomas Brown also discussed Hume's principles of association in Lectures on the Philosophy of the Human Mind (1820).

By the middle of the 19th century, two short books devoted to Hume's theory of causality had appeared, namely, Mary Shepherd's Essay upon the Relation of Cause and Effect (1824) and George Tucker's Essay on Cause and Effect (1850). Although both of these works are critical of Hume's theory, the Humean view of causality became more widely adopted in other metaphysical discussions of causality. For example, although Mill does not mention Hume by name, he nevertheless clearly espouses a Humean conception of causality in his Examination of Sir William Hamilton's Philosophy:

And how, or by what evidence does experience testify to it [the causation hypothesis]? Not by disclosing any nexus between the cause and effect, any sufficient reason in the cause itself why the effect should follow it. No philosopher now makes this supposition, and Sir W. Hamilton positively disclaims it. What experience makes known is the fact of an invariable sequence between every event and some special combination of antecedent conditions, in such sort that wherever and whenever that union of antecedents exists the event does not fail to occur. [Examination of Sir William Hamilton's Philosophy, 26]
The Humean view of causality received an additional boost from Auguste Comte (1798-1857) in his six-volume Cours de philosophie positive (The Positive Philosophy) (1830–1842). At the outset of that work, Comte acknowledges Hume as one of his precursors. In an 1868 discussion of Comte's work, the Edinburgh Review explains more precisely how Hume fully anticipated Comte's positivism:
This is the method of Positive inquiry now universally recognised in every department of science, although as yet imperfectly carried out in some. It was formally announced by Bacon, and is commonly associated with his name, although in truth it was but imperfectly understood and applied by that great teacher of Method. It received definite impulse from the speculations of Hume, who, carrying to their legitimate conclusions the philosophy of his day, showed that we could get nothing from nature, or sense-experience, but ideas of coexistence and a succession; or, in other words, of facts, and the sequences which connect them; and who attempted to prove that this was equally true of the world of mind as of matter. From the one realm as well as the other he cast out all ideas of substance and cause, and left nothing but phenomena and their relations of association. Hume is, therefore, the principal precursor of Comte, as he himself acknowledges. He anticipated to the full the fundamental principle of the Comtean philosophy. He did more than this. For he saw clearly the use that could be made of it polemically; the sceptical or negative bearings of the principle are equally to be found in his writings. So far, therefore, there is nothing original in Positivism. The Scottish sceptic had already anticipated the nature of its attacks against theological philosophy. [Edinburgh Review, April 1868, Vol. 127, p. 322]
As history of philosophy survey books appeared in the second half of the 19th century, Hume found a place in the development of metaphysics, typically standing between the great figures of Berkeley and Kant. An example of this is George Henry Lewes's Biographical History of Philosophy (1873), which devotes a lengthy chapter to Hume. Discussions of Hume also appeared in more specialized histories of philosophy, such as James McCosh's The Scottish Philosophy (1875) and Leslie Stephen's History of English Thought in the Eighteenth Century (1876).
Towards the end of the 19th century, academic writings in the history of philosophy became more "scholarly" in the sense that we understand that term today. T.H. Green wrote a detailed, 400-page study of Hume's Treatise, which was published as an introduction to the edition of Hume's Works (1874), edited by Green and Thomas Grose. Shortly after, three introductory books on Hume's philosophy appeared that contained chapters on Hume's metaphysics, namely, Thomas Huxley's Hume (1879), William Knight's Hume (1886), and Henry Calderwood's David Hume (1898). As academic philosophy journals emerged, scholarly articles on Hume appeared, such as those by J.A. Cain (1885) and William W. Carlile (1896).