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THREE QUESTIONS ABOUT TREATISE I iv 2
Treatise I iv 2, “Of scepticism with regard to the senses,” is a notoriously difficult
section of Hume’s masterpiece.1 In this paper, I examine three puzzling questions that
arise from this section:
(1) Why does Hume think that the “continu’d existence” of sensible
objects and their “existence distinct from the mind” stand or fall together?2
(2) Why does Hume claim that sensible objects obviously have an
intermittent or discontinuous existence?
(3) Why does Hume think that the “coherence” of our impressions (as
opposed to their “constancy”) goes only a little way toward explaining the
belief in body?
1. “Continu’d” and “distinct” existence
Hume says that our belief in the existence of body can be broken down into two
beliefs, namely that
(CE) sensible objects continue to exist while they are not being perceived,
and
(DE) sensible objects exist distinct from (independently of) being perceived.
He immediately adds that if CE is true, then DE is true, and that if DE is true, then CE is
true.3 But while the first of these claims is true, the second one may seem to be quite
1
All page references to the Treatise will be marked as “T” and are to David Hume, A
Treatise of Human Nature, Second Edition, eds. L. A. Selby-Bigge and P. H. Nidditch
(Oxford: Oxford University Press, 1978).
2
Hume uses the term “objects of the senses” rather than the term, “sensible objects.” For
brevity’s sake and for ease of pronunciation, I shall use the latter term throughout this
paper.
3
“These two questions concerning the continu’d and distinct existence of body are
intimately joined together. For if the objects of our senses continue to exist, when they are
not perceiv’d, their existence is of course independent of and distinct from perception;
and vice-versa, if their existence be independent of the perception and distinct from it,
they must continue to exist, even tho’ they be not perceiv’d” (T 188).
2
obviously wrong: why should the “distinct” existence of objects imply their continued
existence? Why couldn’t there be momentary objects, which happen to exist only when
being perceived, but nevertheless do not depend on being perceived for their existence?
Baldly stated, the answer I propose is that if sensible objects existed only when
being perceived, then there would be a constant conjunction between their existence and
their being perceived. So, sensible objects would be causally dependent on being
perceived, in Hume’s sense of causal dependence, and they would accordingly fail to
enjoy a “distinct” existence. So given Hume’s views about causality, DE does imply CE,
just as he says.
But this bald statement is open to the following objection. If sensible objects exist
only when being perceived, this means that if a sensible object exists at time t, then it is
perceived at t; it means that a sensible object’s existence is a sufficient condition for its
being perceived. But clearly this doesn’t show that its being perceived causes it to exist.
Of course, if an object’s existence is a sufficient condition for its being perceived, then its
being perceived is a necessary condition of its existence. But that doesn’t show that the
object’s being perceived causes it to exist: a match’s igniting may be a necessary
condition of its being struck, but its igniting is not a cause of its being struck.
The trouble is that on Hume’s definition of causality, as it is often construed,
perceiving a momentary object cannot be said to cause its existence, anymore than a
match’s igniting causes it to be struck. To see this, consider a schematic formulation of
the first definition of ‘cause’ that Hume gives in the Enquiry:
E1 causes E2 = df
(1) E1 is followed by E2
(2) All events similar to E1 are followed by events similar to E24
4
David Hume, An Enquiry Concerning Human Understanding, Third Edition, eds. L. A.
Selby-Bigge and P. H. Nidditch (Oxford: Oxford University Press, 1975), p. 76.
Subsequent references to this work will be indicated as “E”.
3
In order to allow for cases where cause and effect are simultaneous, I shall replace
“followed by” with “is accompanied by,” and for the sake of simplicity, I shall replace the
notion of similarity with that of sameness in kind (as Hume himself sometimes does).5
This yields the following definition:
D1: E1 causes E2 = df
(1) E1 is accompanied by E2
(2) all events of kind K1 are accompanied by events of kind K2
(3) E1 is of kind K1 and E2 is of kind K2
It is plain that on D1, a cause is a sufficient condition for its effect. So, letting E1 stand for
a momentary object’s existing and E2 stand for its being perceived, the definition
provides no basis for saying that its being perceived causes it to exist, or that its existence
depends causally on its being perceived.
The reply to this objection is that in the Treatise Hume implies, and in the
Enquiry he says, that a cause is not only a sufficient condition for its effect, but also that
it is, in the circumstances at hand, a necessary condition. In the Treatise he says not only
that “the same cause always produce the same effect,” but also that “the same effect never
arises but from the same cause … [so that] where several different objects produce the
same effect, it must be by means of the same quality which we discover to be common
amongst them” (T 173-174). Leaving to one side the questionable claim that alternative
possible causes of a given effect must always have some common feature, this implies
that a given effect can occur, in the circumstances at hand (viz. in the absence of
alternative possible causes of the same effect), only if its actual cause occurs. In the
5
Of course in the Treatise Hume claims that temporal priority of the cause is always
required. But Hume also admits there that his argument for this claim may be
unsatisfactory (T 76), and he does not repeat the claim in the Enquiry. Furthermore, there
are counterexamples to the claim. So I shall it discount here. Two places where Hume
puts his definition in terms of sameness in kind rather than similarity are in the Abstract
of the Treatise, T 649 (“balls of the same kind”) and at E 74 (“But when one particular
species of event always …”). See also T 87 and T 93.
4
Enquiry, Hume famously remarks that one “object” causes another when “if the first
object had not been, the second never had existed” (E 76). Allowing, as we must, for the
possibility of alternative causes, this again means that in the circumstances at hand, the
cause is a necessary condition of its effect. This point can be incorporated into D1 by
expanding clause (2) as follows:
(2A) all events of kind K1 are accompanied by events of kind K2 and all
events of kind K2 are, in the circumstances at hand, accompanied by
events of kind K1.
Thus, for example, to say that striking a match causes it to ignite is to say that all such
strikings are accompanied by ignition, and that in the circumstances at hand (there is no
superheated air present, no other lit match is brought into contact with the one that
ignites, and so forth), all such ignitions are accompanied by strikings.6
Now for purely logical reasons, many philosophers would say that
(a) All cases of an object’s being perceived are accompanied by cases of its
existing.
But for momentary objects, we must also say that
(b) All cases of an object’s existing are, in the circumstances at hand (i.e. given
the oddly perishable nature of such objects), accompanied by cases of its being
perceived.
But this fits the model for saying that momentary objects are caused to exist by being
perceived, in which case Hume is justified in saying that an object with a “distinct” or
“independent” existence cannot be a momentary object--that DE implies CE.
6
I offer a more detailed discussion of how to incorporate into D1 the requirement that a
cause be, in the circumstances at hand, a necessary condition of its effect in my Hume’s
Epistemology and Metaphysics: An Introduction (London and New York: Routledge,
1998), pp, 125-130. The formulation offered there replaces the phrase “in the
circumstances at hand” with a schematic reference to the alternative causes and/or
background conditions that are not present when E1 is the cause of E2.
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It might be objected that if (a) is true for purely logical reasons, then the relation
involved cannot be a causal one. But this objection is not decisive, for consider a parallel
case:
(a1) All cases of murder are accompanied by cases of a person’s dying.
(b1) All cases of a person’s dying are, in the circumstances at hand,
accompanied by cases of murder.
Suppose that, on the model of our expanded D1, we add that
(c) Oswald’s shooting Kennedy was accompanied by Kennedy’s dying.
(d) Oswald’s shooting Kennedy was a case of murder and Kennedy’s
dying was a case of a person’s dying.
The fact that (a1) is logically true does not prevent its being the case that, given the
expanded version of D1, (a1), (b1), (c) and (d) jointly imply that Oswald’s shooting
Kennedy caused Kennedy’s death. The contingent nature of (b1) seems sufficient to
preserve the causal connection between Oswald’s action and Kennedy’s dying. Likewise,
(a)’s being logically true would not go against saying that perceiving a momentary object
causes it to exist, because the contingent nature of (b) seems to preserve the causal
connection between an object’s being perceived and its existing.
2. The intermittent existence of sensible objects
Throughout Treatise I iv 2, Hume asserts that sensible objects have an intermittent
or interrupted existence. His most basic reason for this assertion is that our perceptions
are obviously discontinuous, “broken,” or “interrupted.”7 Consider for example the
7
Occasionally, he supports the assertion with the following Modus Tollens argument (T
210-211, T 214):
(1) If sensible objects continue to exist while not being perceived, then
they exist independently of being perceived.
(2) Sensible objects do not exist independently of being perceived.
__________________________________________________________
... Therefore, sensible objects do not continue to exist while not being
perceived.
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perceptions you have when looking at the furniture in your study. Those perceptions are
plainly discontinuous: they cease to exist each time you glance away from the furniture or
shut your eyes, and for much longer periods of time when you leave your study or are
sound asleep. This obvious point, Hume thinks, incontrovertibly proves the falsity of the
belief in the “continu’d” existence of sensible objects.
To grasp Hume’s thinking, it is crucial to understand that when he says that our
perceptions are interrupted or discontinuous, he is not saying merely that our perceivings,
or acts of perception, or perceptual episodes, are discontinuous, which is of course true.
Rather, he is also claiming that the objects of our perceptions--the things that we
perceive--are discontinuous. For Hume does not distinguish between perceptual episodes
and the objects perceived in them; his use of the term “perceptions” to stand indifferently
for both perceptual episodes and their objects is not just linguistic carelessness, but
embodies a genuine non-recognition of the distinction between them. The only form in
which Hume recognizes a distinction between perception and its objects is that of the
philosophical theory of the “double existence of perceptions and objects” advocated by
Descartes, Locke and others--a theory that Hume rejects. Except within the context of this
theory, Hume sees no place for the distinction between perception and its objects.
Hume’s rejection of this distinction calls for an explanation, if only because the
distinction is so elementary. Even ordinary, unsophisticated common sense would
distinguish between perceiving something, or the perception of something, and what is
perceived, e.g. between seeing an apple, and the apple which is seen. Why then does
Hume reject this elementary distinction?
To support this argument’s second premise, Hume cites the “experiments, which
convince us, that our perceptions are not possest of any independent existence,” such as
pressing one eye with a finger and seeing everything double (T 210). The obvious
interruptedness or discontinuity of our perceptions, however, is Hume’s fundamental
reason for holding that the objects of sense have a discontinuous existence.
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One possible answer lies in Hume’s acceptance of the view that we perceive only
our own impressions.8 For impressions have exactly the same temporal characteristics,
including notably the same duration, as perceptions, taken as perceptual episodes or acts
of perceiving. So the thesis that we perceive only impressions implies that the objects of
perception last no longer than the perceptions themselves. But in that case, the distinction
between perceptions and objects collapses, at least for an empiricist like Hume. For from
Hume’s empiricist point of view, the only thing that could have differentiated between
perceptions and their objects is a temporal difference between them, because any “act” of
perception distinct from the object perceived would not be something of which we could
have any impression: it would be, so to speak, diaphanous or “transparent,” so that
Hume’s empiricism would banish it as a meaningless notion. Thus, it may seem that
Hume’s acceptance of the philosophical view that we perceive only our own impressions
is what explains his otherwise puzzling denial of the elementary distinction between
perceptions and objects, which in turn explains his view that sensible objects have an
intermittent existence.
This explanation, however, puts the cart before the horse. For as I have said,
Hume regards the discontinuity of our perceptions, together with the collapse of any
distinction between perceptions and its objects, as an incontrovertible proof that the
objects of perception are discontinuous. But of course our sense impressions are
discontinuous; so an explanation of why Hume holds that sensible objects are
discontinuous that appeals to his view that we perceive only impressions renders his
claim that perceptions are discontinuous and his rejection of the perception/object
distinction completely otiose, as a proof of the discontinuity of sensible objects: one need
not offer any argument at all to prove that sensible objects are discontinuous, if one
assumes from the start that those objects are only impressions. Hence, in order for
8
This answer is the one I proposed in my Hume’s Epistemology and Metaphysics: An
Introduction, pp. 167-168. For the reason I am about to give, I no longer think that this
answer is satisfactory.
8
Hume’s claim that perceptions are discontinuous, together with his rejection of the
perception/object distinction, to serve as a (non-otiose) argument for his claim that
sensible objects are discontinuous, neither premise of that argument can rest on the view
that we perceive only impressions. So we need a reason for the rejection of the
perception/object distinction that does not rest on that view.
I think that such a reason can be given, by pressing the point that there is no
impression of any introspectible “act of perceiving” that could distinguish it from its
object. For what then does distinguish perceptions from objects? It still seems that the
only thing that could do so would be their temporal characteristics: objects can, so to
speak, outlast the perceptions of them, and so must be distinct from those perceptions.
The central importance of this point is recognized by Kant in his Analogies of
Experience, where he explicitly distinguishes between the time-relations of perceptual
episodes and the time-relations of their objects, saying that while perceptual episodes are
always successive, their objects may be co-existent; for example that while the
perceptions of the front of a house may occur before perceptions of the back, the front
and back of the house that the observer successively sees co-exist. Hume recognizes the
point in his own way, since he says that one of the chief components in our belief in body
is precisely that bodies continue to exist unperceived. But unlike Kant, Hume does not
argue that assigning a different set of time-relations to objects of perception from those of
the perceptions themselves is a condition of the very possibility of experience, and is
therefore epistemologically warranted. Rather, Hume sees the time-relations of perceptual
episodes--what Kant calls the order of apprehension and his commentators usually call
the subjective time-order--as the only one that is necessary for experience: what Kant
considers to be the objective time-order enters into Hume’s thought only as the fictional
product of the imagination working in accordance with merely contingent principles. So,
these time-relations cannot, for Hume, be appealed to as an epistemically warranted way
of distinguishing between perceptions and objects: objects do not knowably have different
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time-relations than perceptions. Within the range of what can be known, there are only
the time-relations of the perceptions themselves, and since there is accordingly no other
set of time-relations to appeal to, and yet no way to distinguish between perceptions and
objects other than their time-relations, the distinction between perceptions and objects
collapses.
3. The role of “coherence”
Hume finds two features of our impressions that explain why we believe in the
continued existence of sensible objects despite their intermittent existence: constancy and
coherence. But he thinks that coherence alone is “too weak to support so vast an edifice,
as is [belief in] the continu’d existence of all external bodies” (T I iv 2, 198-199), and he
places much more weight on his complicated “system” about the way constancy leads to
the belief in object-continuity (T 199-210). Why does Hume think that coherence alone
goes so little way toward explaining this belief?
Here is one of Hume’s characterizations of coherence:
Bodies often change their positions and qualities, and after a little absence
or interruption may become hardly knowable. But here ‘tis observable, that
even in these changes they preserve a coherence, and have a regular
dependence on each other; which is the foundation of a kind of reasoning
from causation, and produces the opinion of their continu’d existence.
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: 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. This coherence, therefore, in their
changes is one of the characteristics of external objects, as well as their
constancy (T I iv 2, 195).
In this passage, Hume’s intention is to describe the kind of uniformity in our impressions
that he calls “coherence.” But as commentators have pointed out, he fails to confine
himself to impressions, and carelessly attributes coherence to “bodies” and “external
objects” themselves.9
See Barry Stroud, Hume (London, Boston and Henley: Routledge & Kegan Paul, 1977),
p. 100 and Jonathan Bennett, Locke, Berkeley, Hume: Central Themes (Oxford: Oxford
University Press, 1971), p. 323.
9
10
Mindful of this point, let us try to characterize coherence without assuming the
existence of physical things. We may define coherence this way: to say that a set of
impressions I exhibits coherence is to say that I is composed of a discontinuous set of
impressions whose members occur in the same temporal relations as the resembling
members of continuous but altering sets of impressions that closely resemble each other
(that exhibit what Hume calls “constancy”). Here is an example, based on Hume’s
illustration. Suppose I have often had the experience that I would call continuously
viewing a slowly diminishing fire in my fireplace. Then, when I have an experience that I
would call discontinuously viewing such a fire--i.e. of seeing the fire, then leaving the
area for a certain period of time, then returning and seeing the fire again--the relations
between the elements of that experience are the same as the relations between the
“matching” elements of the continuous experiences: for example, the impressions of a
fire had at the end are impressions of a smaller fire than the impressions of a fire had at
the beginning, not of a larger one or an equally large one. Such coherence among our
impressions, Hume thinks, contributes to the belief in the continued existence of bodies;
in this instance, to the belief that when I have impressions of a large fire in my fireplace,
followed by no impressions of fire, followed by impressions of a small fire in my
fireplace, a slowly-diminishing fire exists unperceived in between.
Hume initially says, as we saw, that this belief is then based on a “kind of
reasoning from causation.” But a little later he writes that:
But tho’ this conclusion from the coherence of appearances may seem to
be of the same nature with our reasonings concerning causes and effects;
as being deriv’d from custom, and regulated by past experience; we shall
find upon examination, that they are at the bottom considerably different
from each other, and that this inference arises from the understanding, and
from custom in an indirect and oblique manner. For ‘twill readily be
allow’d, that since nothing is ever really present to the mind, besides its
own perceptions, ‘tis not only impossible, that any habit shou’d ever be
acquir’d otherwise than by the regular succession of these perceptions, but
also that any habit shou’d ever exceed that degree of regularity. Any
degree, therefore, of regularity in our perceptions, can never be a
foundation for us to infer a greater degree of regularity in some objects,
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which are not perceiv’d; since this supposes a contradiction, viz. a habit
acquir’d by what was never present to the mind. But ‘tis evident, that
whenever we infer the continu’d existence of the objects of sense from
their coherence, and the frequency of their union, ‘tis in order to bestow on
the objects a greater regularity than what is observ’d in our mere
perceptions. We remark a connexion between two kinds of objects in their
past appearance to the senses, but are not able to observe this connexion to
be perfectly constant, since the turning about of our head, or the shutting
of our eyes is able to break it. What then do we suppose in this case, but
that these objects still continue their usual connexion, notwithstanding
their apparent interruption, and that the irregular appearances are join’d by
something, of which we are insensible? But as all reasoning concerning
matters of fact arises only from custom, and custom can only be the effect
of repeated perceptions, the extending of custom and reasoning beyond the
perceptions can never be the direct and natural effect of the constant
repetition and connexion, but must arise from the co-operation of some
other principles.” (T 197-198)
To understand Hume’s reasoning here, contrast two types of cases:
Type-I cases: I watch a slowly diminishing fire in my fireplace. I neither
shut my eyes nor turn my head away from the fire, but watch the fire
continuously until it burns itself out.
Type-II cases: I watch a slowly diminishing fire in my fireplace. However,
at one or more times while the fire is burning, I shut my eyes, turn my
head away from the fire, or leave the room.
Assuming that I have experienced several type-I cases, my visual impressions in a type-II
case exhibit coherence: discontinuous impressions of a fire occur in the same temporal
relations as the resembling members of the impressions I obtained in the type-I cases.
Suppose that on the basis of this coherence, I infer that in a type-II case, I am observing a
fire that continues to burn while my eyes are shut, my head is turned away, or I am absent
from the room. Our question is this: why does Hume think I am then making an inference
that does not conform to the kind of custom-based causal reasoning which he holds to be
the sole basis of any knowledge of matters of fact not based on present perception or on
memory?
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To see why, let us first imagine that a constant-conjunction-establishing series of
type-I cases occurs in my experience--i.e. that I have the experience of continuously
watching a diminishing fire often enough to establish a constant conjunction between
visual impressions like those I have at the first moment (= time-interval) of watching the
fire and visual impressions like those I have at the second moment, and between visual
impressions like those I have at the second moment and visual impressions like those I
have at the third moment, and so on. Notice, then, that in a new case where I have
impressions like those I have repeatedly had at the first moment of watching the fire, I can
infer, according to Hume’s theory of causal-inductive inference, that I will next have
impressions like those I have repeatedly had at the second moment; likewise, in a new
case where I have impressions like those I have repeatedly had at the second moment of
watching the fire, I can infer that I will next have impressions like those I have repeatedly
had at the third moment; and so forth.
Now imagine that a type II case occurs in my experience: I watch a fire in my
fireplace for the first moment; then shut my eyes, turn my head away, or leave the room
for the second moment; then watch the fire again for the third moment; then shut my
eyes, turn my head away, or leave the room for the fourth moment; and so on. Then the
coherence of my impressions causes me to infer that even during the second moment and
the fourth moment, something just like the visual impressions I had during the second and
fourth moments of the type-I cases existed, albeit unperceived by me. But clearly enough,
this inference is not licensed by the custom-based reasoning from constant conjunction
that Hume discusses. For there is a difference between the type-I cases I remember and
the type-II case I have just experienced: in the former, each visual impression of a fire
was immediately followed in time by another visual impression of a fire; in the latter,
each visual impression of a fire is immediately followed in time by other kinds of
impressions. For the inference performed in case II to be a custom-based causal inference,
there would have to be a constant conjunction between the visual impressions obtained at
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the first moment of the type-I cases and those obtained at the second moment of the typeII cases, and between the visual impressions obtained at the second moment of the type-I
cases and those obtained at the third moment of the type-II cases, and so on. But there is
no such constant conjunction; for in all type-II cases my visual impressions of a fire at
one moment are followed at the next moment by visual impressions of something else,
and so by a “gap” in the impressions of a fire. Therefore, when I infer the existence, in a
type-II case, of something just like the visual impressions I have following each visual
impression of a fire in type-I cases, I am attributing a greater degree of regularity or
uniformity to the world than I ever perceive. And this illustrates why coherence alone is
“too weak to support so vast an edifice, as is [belief in] the continu’d existence of all
external bodies.”
It might be objected that the reasoning I have attributed to Hume is mistaken,
because if a constant-conjunction establishing set of type-I cases has already occurred in
my experience, then in any new case where I experience the impressions I had at moment
M of such cases, I can infer the existence of what I perceived at moment M + 1. So if,
say, five minutes into type-I cases I had impressions of a large fire and a minute later of a
smaller fire, then in any other similar case where I have impressions of a large fire I can
infer that a minute later a smaller fire will exist, whether I perceive it or not.
But this objection fails, because the situation just described is not a case of
coherence at all. For in this situation no type-II case has occurred as yet; such a case will
only have occurred after I have had the impressions of a large fire, followed by no
impressions of a fire, followed by impressions of a smaller fire. But once such a case has
occurred, it ipso facto constitutes an exception or counterexample to the experienced
regularity found in the type-I cases alone. Thus again, to suppose that something like
what I experienced at moment M + 1 of the type-I cases exists at moment M + 1 of the
type-II cases is to attribute a greater degree of regularity to the world than I perceive. It is
true that type-II cases will also include certain impressions, associated with turning my
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head away, shutting my eyes, or leaving the room, that mark them off from type-I cases.
But this does not salvage the objection. For the occurrence of these other impressions
does not re-establish a constant conjunction between impressions like those had at
moment M and those had at moment M + 1 of the type-I cases.
Georges Dicker
SUNY Brockport
gdicker@brockport.edu