Hume's targets in part IV, section II, of book I of his Treatise on Human Nature, 1 entitled "Of scepticism with regard to the senses," are two theses, each concerning the objects of perception, that Hume holds to be "intimately connected together" (188) in that one is true if and only both are. The first thesis I will call the Continuity Thesis: the objects of perception continue to exist even when they are unperceived. The second I will call the Independence Thesis: the objects of perception are ontologically independent of the mind, and could exist even if there were no minds. In this paper, I will first present Hume's arguments that these two Theses cannot be arrived at in an epistemologically legitimate manner. I will then examine Hume's account of how we do arrive at these Theses. Although Hume's arguments that the Continuity and Independence Theses cannot be arrived at in a epistemologically legitimate manner, his account of how we do arrive at the Theses will be found wanting.
The Continuity Thesis and the Independence Thesis are not quite as intimately connected as Hume thinks they are. Perhaps the objects of perception are ontologically independent of the mind, but nevertheless, by an utter coincidence, they cease to exist whenever they cease to be perceived. The strongest conclusion we could derive from the Independence Thesis alone is that it is possible that the Continuity Thesis is true, for in a world with no minds, nothing is perceived. Similarly, the Continuity Thesis by itself will not suffice to establish the Independence Thesis. Perhaps the objects of perception are dependent on minds, but all that is required for them to continue to exist is that they are thought about. Nevertheless, Hume is right that the two theses are intimately linked. It would seem rather strange for mere thought, rather than the stronger relation of perception, to have the ability to maintain an object in existence, given the possibility of mistaken beliefs about the existence of things. And it would be an extraordinary coincidence indeed if the objects of perception just happened to go out of existence whenever they cease to be perceived, given that there is no ontological connection between the objects of perception and the mind.
There are two sorts of persons who hold the Continuity Thesis and the Independence Thesis: those whom Hume calls "the vulgar," and those whom Hume calls "philosophers" (192). The difference between the vulgar and the philosophers is in how they understand the phrase "the objects of perception" in the two Theses. The vulgar are those whom we might now call "naive realists," or less prejudicially, "direct realists." The vulgar, according to Hume, make no distinction between perceptions, which Hume calls "impressions," and the objects of perception. "Those very sensations, which enter by the eye or ear, are with them the true objects. . . " (202). By "the philosophers," Hume means Locke and Descartes, who hold a doctrine of "double existence" (189): whenever I have a perception, there is a wholly mental entity, which Descartes would call a "mode" of the mind; and, at least whenever the perception is veridical, there is an extra-mental entity that is represented by the mental entity, and which resembles the mental entity, at least in those qualities that Locke calls "primary," such as shape, size, and relative spatial position. It is these latter extra-mental objects that are "the objects of perception" that the philosophers believe continue to exist when unperceived, and exist independently of the mind.
Hume's strategy of attack against the two Theses is an examination of their psychological origin. Hume holds that there are three sorts of mental faculties, or processes, by which beliefs can arise: sensation, reason, and imagination. The first two of these produce justified beliefs. Although Hume would not call all the true beliefs produced by sensation or reason "knowledge," requiring that a belief attain Cartesian certainty in order to merit that appellation, 2 the beliefs produced by those faculties or processes are in some way legitimate or proper, in a way that those produced by the imagination are not. The faculty of imagination, on the other hand, is for Hume, as it was for Spinoza, the faculty of spurious inferences; the beliefs formed by the imagination do not have the epistemological legitimacy that beliefs arising through sensation or reason do. Hume's argument against the Continuity Thesis and Independence Thesis is not a direct one to their falsity; it is rather an argument that we have no good reason to suppose that the Theses are true, for they arise from the faculty of imagination. Hume's strategy in part IV, section I, is to examine both the Continuity Thesis and the Independence Thesis, both as understood by the vulgar and as understood by the philosophers, to see if they do, or even could, arise from sensation or reason. He concludes they do not, and so must arise at least partly from imagination, and are thus tainted with epistemological illegitimacy.
Hume begins by examining whether sensation can establish either the Continuity Thesis or the Independence Thesis. Hume notes immediately that sensation is incapable of establishing the Continuity Thesis directly:
To begin with the SENSES, 'tis evident these faculties are incapable of giving rise to the notion of the continu'd existence of their objects, after they no longer appear to the senses. For that is a contradiction in terms, and supposes that the senses continue to operate, even after they have ceas'd all manner of operation. (188)To perceive that an object exists, I must perceive that object; thus I cannot perceive that an unperceived object exists. Thus if sensation is to establish the Continuity Thesis, it must do so indirectly, by establishing the Independence Thesis, from which we may infer the Continuity Thesis.
More effort is required on Hume's part to argue that the Independence Thesis cannot be established by sensation alone. The text in this section is rather disorganized, but if we are careful we can distinguish four different arguments against the possibility that sensation alone could establish the Independence Thesis. Two of these arguments appear to be directed at the vulgar only, one is directed against the philosophers only, and one is directed against both the vulgar and the philosophers. For expository purposes, I will consider the arguments in a different order than they appear in the text.
The first argument I will consider is the last Hume gives against the possibility that sensation alone can establish the Independence Thesis, and is directed against both the vulgar and the philosophers. Hume notes that both the philosophers and the vulgar make a distinction that Locke calls the distinction between "primary" and "secondary" perceptible qualities.
. . . There are three different kinds of impressions convey'd by the senses. The first are those of figure, bulk, motion and solidity of bodies. The second those of colours, tastes, smells, sounds, heat and cold. The third are pains and pleasures, that arise from the application of objects to our bodies. . . . Both philosophers and the vulgar suppose the first of these to have a distinct [independent] continu'd existence. The vulgar only regard the second as on the same footing. Both philosophers and the vulgar, again, esteem the third to be merely perceptions; and consequently interrupted and dependent beings. (192, comment in brackets added)In Locke's terminology, the philosophers hold that only the first group are primary qualities, having some real existence in the objects of perception, and the second and third groups are merely secondary qualities, existing only through the interaction of minds and bodies. Thus, according to Locke, things cease to be colored whenever they cease to be perceived: "Can anyone thing any real alterations are made in the porphyry by the presence or absence of light; and that those Ideas of whiteness and redness are really in the porphyry in the light, when it is plain it has no colour in the dark?" 3 The shape and size of perceivable objects, on the other hand, does not change merely in virtue of the relationship of the object to a perceiver, according to Locke.
Despite Locke's apparent incredulity at the thought that porphyry continues to be colored red and white even in utter darkness, the natural, unphilosophical attitude, i.e., the attitude of the vulgar, is to hold that objects are colored even when they are not visually perceived. What could be more natural than to say "It was dark, so I couldn't tell what color dress she was wearing," rather than "It was dark, so her dress had no color"? And this extends to other sense perceptible qualities as well; we say "The concert was noisy, but it did not seem so, since I wore ear plugs," but we do not say "I wore ear plugs, so the concert was not noisy." But Hume is correct that even the vulgar do not extend the attribution of objective reality to all sense-perceptible qualities. We may naturally say "The colors of the sunset were beautiful, but Mary, who is color-blind, could not see them"; but it would be seem contradictory to say "The dental operation was quite painful, but John was anesthetized, and so could not feel it."
At this point in Hume's exploration of possible justifications for the Continuity Thesis and the Independence Thesis, Hume has no need to argue that this distinction between primary and secondary qualities can play no role in establishing the two Theses. (Indeed, later, Hume's account of how the imagination arrives at the two Theses will also, he claims, explain the primary/secondary quality distinction, although it will not provide any epistemic justification for it.) For at the moment, Hume is only interested in establishing that sensation alone cannot establish the Independence Thesis (having argued earlier that sensation cannot establish the Continuity Thesis). If the primary/secondary quality distinction were the route via which sensation alone established the Independence Thesis, sensation alone must be able to make the primary/secondary distinction. This, Hume claims, cannot be done, although vulgar opinion sometimes holds that it can.
So strong is the prejudice for the distinct continu'd existence of the former [primary] qualities, that when the contrary opinion is advanc'd by modern philosophers, people imagine they can almost refute it from their feeling and experience, and that their very senses contradict this philosophy. (192, comment in brackets added) There is, however, Hume claims, no relevant qualitative (i.e., directly sense-perceptible) distinction between the qualities that either the vulgar or the philosophers consider to be "primary" and those they consider to be "secondary."
Now 'tis evident, that, whatever our philosophical opinion, colours, sounds, heat and cold, as far as appears to the senses, exist after the same manner with motion and solidity, and that the difference we make betwixt them in this respect, arises not from the mere perception. . . . 'Tis also evident, that colours, sounds, &c. are originally on the same footing with the pain that arises from steel, and pleasure that proceeds from a fire. . . . (192)Thus our very natural distinction between primary qualities, having existence independent of a perceiving mind, and secondary qualities, that exist "only in the mind of the perceiver," cannot be made on the basis of sensation alone; therefore the primary/secondary quality distinction cannot be the route via which the Independence Thesis is established by sensation alone.
The second argument I will examine that Hume gives against the possibility that the Independence Thesis can be established by sensation alone is one that is directed primarily at the vulgar. The way in which Hume imagines the vulgar arguing is a quite natural one, and is as follows: By sensation alone I establish that the objects of perception are situated in space externally to my mind; and if two things are situated in space externally to one another, they are ontologically independent of one another, that is, it is metaphysically possible that one exist and the other not; therefore, sensation alone establishes that the objects of perception are ontologically independent of my mind. Hume has no wish to deny the major premise of this argument, that things situated in space external to one another are ontologically independent of one another. Hume himself uses this principle earlier in the Treatise, when he argues against the demonstrability of the Principle of Universal Causation, the thesis that every event has a cause:
Now that the latter proposition [the Principle of Universal Causation] is utterly incapable of demonstrative proof, we may satisfy ourselves by considering, that as all distinct ideas [ideas that are situated externally to one another in perceptual space] are separable from each other, and as the ideas of cause and effect are evidently distinct, 'twill be easy for us to conceive any object to be non-existent this moment, and existent the next, without conjoining to it the distinct idea of a cause or productive principle. The separation, therefore, of the idea of a cause from that of a beginning of existence, is plainly possible for the imagination; and consequently the actual separation of these objects is so far possible, that it implies no contradiction nor absurdity. . . . (79-80, comments in brackets added) 4In this passage, Hume argues against the necessity of a cause for every effect as follows: any event that is a cause is spatially external (in perceptual space) to its effect; since they are spatially external, they are ontologically independent of one another; therefore the event that is effect could exist without a cause.
We might call this principle, that objects and events that are spatially external to one another are ontologically independent of one another, the "Cut and Paste Principle." Metaphorically, if we have two possible worlds, we may rearrange their constituent spatio-temporal parts as we please by "cutting and pasting," and the result will also be a possible world. (Since the "empty world," in which there are no objects or events, is also a possible world, we may paste any spatio-temporal part of the empty world "in front of" an event of the actual world, and the resulting world will be one in which the event has no cause.)
Since Hume cannot consistently deny the Cut and Paste Principle, which is used as the major premise in the argument under consideration that sensation alone establishes the Independence Thesis, he must attack the minor premise, which asserts that sensation alone establishes that the objects of perception are spatially external to my mind. Hume imagines someone arguing for the minor premise of the argument as follows:
[Sensation alone informs me that] The paper, on which I write at present, is beyond my hand. [Sensation alone informs me that] The table is beyond the paper. [Sensation alone informs me of] The walls beyond the table. And in casting my eye towards the window, I perceive a great extent of fields and buildings beyond my chamber. From all this it may be infer'd, that no other faculty is requir'd, beside the senses, to convince of the external existence of body. (190-1, comments in brackets added)Hume has three things to say against such an argument, none of which are entirely satisfactory. Indeed, the first thing he says is downright unfair and inconsistent with his own final position: "First, That, properly speaking, 'tis not our body we perceive, when we regard our limbs and members, but certain impressions, which enter by the senses" (191). Hume here is rallying the philosophers, who believe in a "double existence" of perception and object perceived, against the argument in question. But the vulgar, who are presumably the ones who are making the argument in question, do not believe in this "double existence": they make no distinction between perceptions and objects perceived. For that matter, Hume himself does not believe in this distinction either; he we later in this section call the Lockean/Cartesian doctrine of double existence "the monstrous offspring of two principles, which are contrary to each other, which are both at once embrac'd by the mind, and which are unable mutually to destroy one another" (215). Hume's claim here would be a successful ad hominem consideration if it were the philosophers who were offering this argument, but presumably it is not, for this argument would be inconsistent with Descartes' first Meditation, in which he argues that sensation alone cannot establish the existence of a spatially external world.
The second and third things Hume says about this argument of the vulgar should be looked at together.
Secondly, Sounds, and tastes, and smells, tho' commonly regarded by the mind as continu'd independent qualities, appear not to have any existence in sensation, and consequently cannot appear to the senses as situated externally to the body. . . . Thirdly, Even our sight informs us not of distance or outness (so to speak) immediately and without a certain reasoning and experience, as is acknowledg'd by the most rational philosophers.By "the most rational philosophers," Hume presumably has in mind Berkeley, whose New Theory of Vision first appeared thirty years prior to the publication of the Hume's Treatise, and who held that the world appears as two-dimensional to our visual faculty, and we infer its three-dimensionality through experience. Hume's remarks about smells and tastes are presumably correct, at least for human beings, although other mammals seem to have a sense of "directionality" to their olfactory sensations. His claim about sound is probably not entirely accurate: we say that something sounds "a long way off," and we can sometimes tell what direction a sound is coming from. And Hume's claim about vision is called into question by the experimental finding that even newborn infants respond to "looming" images, ones in which it appears as if something is coming directly towards the perceiver. But note that even if Hume's claims were right, that neither smells, tastes, sounds, nor sights appear to pure sensation as if they are spatially external to the perceiver, Hume for unknown reasons does not consider the sense of touch. Surely if any sense gives us direct knowledge of the externality of the objects of perception, it is touch.
Although the arguments of Hume's I have considered so far against the possibility that sensation alone can establish the Independence Thesis, the final two I will consider, which are the first two Hume gives, and are directed against the philosophers and the vulgar respectively, are much stronger: decisive, perhaps, for the argument directed against the philosophers. Hume argues thus against the possibility that sensation alone could establish that the objects of perception are independent of the perceiving mind, given the philosopher's understanding of "objects of perception":
. . . They [the senses] convey to us nothing but a single perception, and never give us the least intimation of any thing beyond. A single perception can never produce the idea of a double existence, but by some inference either of the reason or imagination. When the mind looks farther than what immediately appears to it, its conclusions can never be put to the account of the senses; and it certainly looks farther, when from a single impression it infers a double existence, and supposes the relation of resemblance and causation betwixt them. (189)"How," Hume asks a hypothetical philosopher (certainly not Descartes) who claims that sensation alone establishes that the doctrine of double existence, "can you establish on the basis of sensation alone the existence of something (a) that is not identical to your perception, (b) that resembles your perception, at least in those qualities you call "primary," and (c) is the cause of your perception?" Seemingly the only way would be some sort of causal reasoning: but causal reasoning is a species of reasoning, and in utilizing causal reasoning you will have used something other than your faculty of sensation. This argument of Hume's seems to me decisive against anyone who might claim that sensation alone can establish the doctrine of double existence, which is a crucial part of the philosopher's understanding of the Independence Thesis.
Hume's best argument against the vulgar position is an interesting one, in that it reveals that the position has a substantial metaphysics of the mind lurking in its corners, and gives us the first taste of Hume's own metaphysics of mind, his "bundle theory," fully spelled out later in part II, section VI. The vulgar, unlike the philosophers, make no distinction between the perceptions and the objects of perception. In order to hold the Independence Thesis, then, the vulgar must make a distinction between the mind and its perceptions. (The philosophers need not make this distinction in order to consistently hold the Independence Thesis.) "Now if the senses presented out impressions as external to, and independent of ourselves, both the objects and ourselves must be obvious to our senses, otherwise they cou'd not be compar'd by these faculties. The difficulty, then, is how far we are ourselves the objects of our senses" (189). Saying this, Hume pushes the issue aside until section VI, in which he argues that we have no perception of a "self" distinct from our perceptions. Indeed, Hume argues later, "what we call a mind, is nothing but a heap or collection of different perceptions, united together by certain relations, and suppos'd, tho' falsely, to be endow'd with a perfect simplicity and identity" (207).
Putting together these last two arguments into one, we may construct a powerful argument against the possibility of establishing the Independence Thesis by sensation alone. (1) To establish the Independence Thesis, we must establish either (a) a distinction between perceptions and the objects of perception, or (b) a distinction between perceptions and the mind that has those perceptions. (2) Sensation alone cannot establish a distinction between perceptions and the objects of perception. (3) Sensation alone cannot establish a distinction between perceptions and the mind that has those perceptions. Therefore, (4) sensation alone cannot establish the Independence Thesis.
In contrast to the care Hume takes to argue against the possibility that sensation alone can establish the Continuity and Independence Theses, Hume is disappointingly quick with his dismissal of the possibility that the faculty of reason could aid in establishing the Independence Thesis. Hume seems to be satisfied in claiming merely that we do not establish either Thesis by use of reason, and does not bother with any argument that we cannot establish either Thesis. ". . . Indeed, whatever convincing arguments philosophers may fancy they can produce to establish the belief of objects independent of the mind, 'tis obvious these arguments are known but to very few, and that 'tis not by them, that children, peasants, and the greatest part of mankind are induc'd to attribute objects to some impressions [the primary qualities], and deny them to others [the secondary qualities]" (193, comments in brackets added). He does claim that "all the conclusions, which the vulgar form on this head, are directly contrary to those, which are confirm'd by philosophy" (ibid.). Here Hume suggests that the philosophers have a decisive argument against the Independence and Continuity Theses as understood by the vulgar, who make no distinction between perceptions and the objects of perception; presumably it follows from this that reason establishes that the Theses are false as understood by the vulgar. Unfortunately, Hume does not here tell us what this argument of the philosophers is, although later he does offer an argument that might be what he is referring to:
When we press one eye with a finger, we immediately perceive all the objects to become double, and one half of them to be remov'd from their common and natural position. But as we do not attribute a continu'd existence to both these perceptions, and as they are both of the same nature, we clearly perceive, that all our perceptions are dependent on out organs, and the disposition of our nerves and animal spirits. This opinion is confirm'd by the seeming encrease and diminution of objects, according to their distance; by the apparent alterations in their figure; by the changes in their colour and other qualities from our sickness and distempers; and by an infinite number of other experiments of the same kind; from all which we learn, that our sensible perceptions are not possest of any distinct or independent existence. (210-1) Here Hume might as well have written, "Read Berkeley, and you will be convinc'd that the objects of immediate awareness are not independent of the mind."
Further, Hume claims that the philosophers' independent and continuous objects of perception, distinct from the perceptions themselves, cannot be supported by reason either: "Even after we distinguish our perceptions from our objects, 'twill appear presently, that we are still incapable of reasoning from the existence of one to that of the other. . ." (193). Unfortunately, the promised argument never appears, as the rest of the section is devoted to an explanation of how we actually come to hold the Continuity and Independence Theses; but it seems clear what Hume's argument would be. In part III, section II, Hume makes clear the reason for his interest in causation by saying, " 'Tis only causation, which produces such a connexion, as to give us assurance from the existence or action of one object, that 'twas follow'd or preceded by any other existence or action" (73-4). And although it is often said that Hume is a skeptic about causation, this is true only insofar as Hume regards causal inferences as less than certain (and he is correct in so regarding them), and he requires certainty for knowledge. Causal reasoning is a legitimate, epistemologically acceptable mode of reasoning for Hume; and as Hume makes clear in the preceding quote, he holds that it is the only mode of reasoning that can allow us to legitimately infer the existence of one object from the existence of another. We are given the existence of our perceptions; if we can legitimately go beyond our perceptions, as the philosophers do in their doctrine of double existence, it must be by causal reasoning. But, as Hume argues in part III, section XIV, the establishment of a causal relation requires observation of constant contiguity of similar objects, such as when we infer that wood-fires cause smoke from the observation that burning wood is invariably accompanied by smoke. But according to the philosophers, all we ever actually observe are our perceptions, and never directly observe the independently existing objects that cause them. Causal inference goes beyond the direct evidence of the senses, but must be grounded in sensation, through observation of both the sorts of entities that are considered causes and those considered effects; but by hypothesis the independent objects of perception are never observable, and so causal reasoning cannot establish their existence. The doctrine of double existence can never be established on the basis of causal reasoning, and since causal reasoning is the only sort of reasoning that could establish this doctrine, reason cannot establish the doctrine of double existence.
Since the Continuity and Independence Theses cannot be established by sensation alone or by reason in conjunction with sensation, it must be the case that we arrive at the Theses through an error of the imagination. The rest of part IV, section II is devoted to Hume's account of how this comes about. Hume begins by turning to our natural distinction between primary and secondary qualities to find out how imagination leads us to this conclusion.
Since all impressions are internal and perishing existences, and appear as such, the notion of their distinct and continu'd existence must arise from a concurrence of some of their qualities with the qualities of the imagination; and since this notion does not extend to all of them, it must arise from certain qualities peculiar to some impressions. (194) Hume, correctly rejecting the hypothesis that it is the "involuntariness" or the "force and vivacity" of impressions that makes us (mistakenly) believe that they have some objective, mind-independent existence, claims to find two qualities, or rather relations, that those qualities that the vulgar consider primary have in common: coherence and constancy (194-5). Constancy is a similarity relation, which he describes as follows: "These mountains, and houses, and trees, which lie at present under my eye, have always appear'd to me in the same order; and when I lose sight of them by shutting my eyes or tuning my head, I soon after find them return upon me without the least alteration" (194). Coherence is a relation that applies not to individual perceptions, but to sequences of perceptions: "When I return to my chamber after an hour's absence, I find not my fire in the same situation, in which I left it [and thus there is no constancy]; But then I am accustom'd in other instances to see a like alteration produc'd in a like time, whether I am present or absent, near or remote" (195). Here we find Hume at his most Kantian. Kant argues in the Trancendental Aesthetic of the Critique of Pure Reason that it is only because of spatially and temporally ordered perceptions that we have belief in an objective world. Hume, however, does not go on as Kant does, in the Refutation of Material Idealism, to argue that it is only in light of our belief in an objective world that we can be said to have any knowledge at all, even first-person knowledge of our own mental states. Whereas Kant sees the spatio-temporal order of our perceptions at the foundation of our knowledge, Hume regards it as the source of a massive illusion, albeit one that even he cannot overcome when he stops doing philosophy and returns to everyday life.
Hume's story about how coherence is leads to the supposition of continuous and mind-independent objects is an interesting one, but for the purposes of this paper I will focus on the role Hume assigns to constancy, as Hume gives it the primary role in producing in us the Continuity and Independence Theses: "But whatever force we may ascribe to this principle [that the imagination infers unperceived continuants in order to increase the level of coherence], I am afraid 'tis too weak to support alone so vast an edifice, as is that of the continu'd existence of all external bodies; and that we must join the constancy of their appearance to the coherence, in order to give a satisfactory account of that opinion" (198-9). (As we shall see, one of the criticisms I will make of Hume's account of how the imagination arrives at the Continuity and Independence Theses will apply equally to an account that includes a strong role for coherence.)
Constancy leads us first to the Continuity Thesis, and from there we infer the Independence Thesis, according to Hume: "This inference from the constancy of our perceptions . . . gives rise to the opinion of the continu'd existence of body, which is prior to that of its distinct existence, and produces the latter principle" (199). The first step in the path that leads our imagination to the Continuity Thesis is the tendency of our imagination to conflate resemblance with strict numerical identity:
When we have been accustom'd to observe a constancy in certain impressions, and have found, that the perception of the sun or ocean, for instance, returns upon us after an absence or annihilation with like parts and in a like order, as at its first appearance, we are not apt to regard these interrupted perceptions as different [i.e., numerically distinct], (which they really are) but on the contrary consider them as individually the same [i.e., numerically identical], upon account of their resemblance. (ibid., comments in brackets added)According to Hume, then, we recognize that strict identity is incompatible with temporal interruption. So in order to reconcile the imagined numerical identity of the earlier and later perceptions with their temporal non-contiguity, reason concludes that the object has continued to exist throughout the interval, even though it was unperceived. So strongly ingrained is this belief in unperceived continuants, Hume claims, that even when philosophers come to recognize, through examples such as Hume's case of the double vision we experience when we apply pressure to the eye, that our perceptions are mind-dependent and discontinuous, they continue to believe in a unperceived continuants, although they make a distinction between the mind-independent objects of perception and the mind-dependent perceptions (211, 215).
There are at least two problems, however, with Hume's account of how the imagination leads us to hold the Continuity Thesis (and thereby to the Independence Thesis). The first of these, perhaps, is a mere gap in the account, a gap that could perhaps be filled in. The problem is this: given that our imagination makes us mistake resembling perceptions for identical ones, what makes us think that the identical objects must be temporally continuous? Why do we hypothesize that if we perceive A at time t1 and again later at time t2, but do not perceive A during some interval between t1 and t2, that it continued to exist during that interval? Why don't we naturally tend toward the hypothesis that A is a recurrent entity, that can exist at one time, cease to exist for a period, and reappear later? There is nothing conceptual about the nature of strict, numerical identity that requires that entities not have an interrupted existence. The hypothesis of recurring entities would still be an error, according to Hume, but it would surely be a more manageable one than the one we actually make. The hypothesis of recurring entities would not make us believe in unperceived entities, and thus it would in any way force us to embrace the Independence Thesis. If reason works to reconcile the errors of the imagination with the observations of the senses, as Hume claims it does, one would suspect that it would arrive at a Recurrence Thesis rather than the Continuity Thesis.
This first problem might be regarded as a gap in the account, to be either explained by further Humean psychology, or, in the worst case, by appeal to a purported brute fact that human minds find recurrence repugnant, but find no such repugnance in continuity. The second problem is more serious, however, and undermines the entire basis of Hume's explanation of how we arrive at the Continuity and Independence Theses. Recall that Hume claimed that it was coherence and constancy that distinguished those sense-perceptible qualities we regard as primary (i.e., continuous and hence mind-independent) from those we regard as secondary (i.e., interrupted and hence mind-dependent). But it seems clear that this line cannot mark our natural, vulgar distinction. Even the paradigm example of a quality we regard as secondary, pain, quite often exhibits constancy and coherence. One often feels pains that are qualitatively identical to previous pains, without ever having a tendency to believe that one pain is numerically identical with another, much less to hold that the pain persisted unperceived during an interval. Pains may exhibit coherence as well. I'm told by a unfortunate friend that migraines have a characteristic pattern over time, and when he begins to feel one coming on, he can take some pain-killers to make it "go away." There is no tendency here to say that the migraine persists unfelt during after he takes the pain-killers. This example demonstrates that it cannot be the relationships of constancy and coherence between sensations in virtue of which we make the natural primary/secondary quality distinction, and hence cannot be what makes us regard the objects of perception as continuous and mind-independent.
I have herein examined Hume's arguments against the possibility that the Continuity and Independence Theses can be established by the epistemologically respectable faculties of sensation and reason. Although Hume's arguments against this possibility are strong, his explanation of how our faculty of imagination causes us to arrive at the Theses was found to be less than adequate. In light of this, it must either be the case that although the imagination is the culprit in leading us to hold the Continuity and Independence Theses, the process by which the imagination leads us to the Theses quite different than the one Hume hypothesized, or there is a flaw in Hume's negative arguments, and it is possible for us to arrive at the Theses by the respectible routes after all.
1. All reference to Hume herein are to book I of A Treatise on Human Nature. New York: Oxford University Press, 1978.
2. Hume seems to argue in part IV, section I, that no belief ever attains Cartesian certainty, for "all knowledge degenerates into probability" (180), due to the fallibility of even the faculties of sensation and reason.
3. An Essay Concerning Human Understading, Vol I. New York: Dover, 1959.
4. It should be noted that, on the interpretation I am giving here, Hume uses the word 'distinct' in a different way in his discussion of the Principle of Universal Causation than he does in his discussion of the Independence Thesis. When he discusses the Principle of Universal Causation, he uses 'distinct' to mean 'situated extenal to one another in perceptual space'; in his discussion of the Independence thesis, he often uses 'distinct' to mean 'ontologically independent'. If he were using 'distinct' in the latter way in his discussion of the Principle of Universal Causation, he would be merely begging the question of the ontological indepedence of causes and effects.
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