## 1. Dreams and epistemology


### 1.1 Cartesian dream skepticism



Dream skepticism has traditionally been the most famous and widely
discussed philosophical problem raised by dreaming (see Williams 1978;
Stroud 1984). In the *Meditations*, Descartes uses dreams to
motivate skepticism about sensory-based beliefs about the external
world and his own bodily existence. He notes that sensory experience
can also lead us astray in commonplace sensory illusions such as
seeing things as too big or small. But he does not think such cases
justify general doubts about the reliability of sensory perception: by
taking a closer look at an object seen under suboptimal conditions, we
can easily avoid deception. By contrast, dreams suggest that even in a
seemingly best-case scenario of sensory perception (Stroud 1984),
deception is possible. Even the realistic experience of sitting
dressed by the fire and looking at a piece of paper in one's hands
(Descartes 1641: I.5) is something that can, and according to
Descartes often does, occur in a dream.



There are different ways of construing the dream argument. A strong
reading is that Descartes is trapped in a lifelong dream and none of
his experiences have ever been caused by external objects (the
*Always Dreaming Doubt*; see Newman 2019). A weaker reading is
that he is just sometimes dreaming but cannot rule out at any given
moment that he is dreaming right now (the *Now Dreaming Doubt*;
see Newman 2019). This is still epistemologically worrisome: even
though some of his sensory-based beliefs might be true, he cannot
determine which these are unless he can rule out that he is dreaming.
Doubt is thus cast on all of his beliefs, making sensory-based
knowledge slip out of reach.



Cartesian-style skeptical arguments have the following form (quoted
from Klein 2015):


1. If I know that *p*, then there are no genuine grounds for
doubting that *p*.
2. *U* is a genuine ground for doubting that *p*.
3. Therefore, I do not know that *p*.



If we apply this to the case of dreaming, we get:


1. If I know that I am sitting dressed by the fire, then there are no
genuine grounds for doubting that I am really sitting dressed by the
fire.
2. If I were now dreaming, this would be a genuine ground for
doubting that I am sitting dressed by the fire: in dreams, I have
often had the realistic experience of sitting dressed by the fire when
I was actually lying undressed in bed!
3. Therefore, I do not know that I am now sitting dressed by the
fire.



Importantly, both strong and weak versions of the dream argument cast
doubt only on sensory-based beliefs, but leave other beliefs
unscathed. According to Descartes, that 2+3=5 or that a square has no
more than 4 sides is knowable even if he is now dreaming:



> 
> 
> although, in truth, I should be dreaming, the rule still holds that
> all which is clearly presented to my intellect is indisputably true.
> (Descartes 1641: V.15) 
> 
> 
> 



By Descartes' lights, dreams do not undermine our ability to engage in
the project of pure, rational enquiry (Frankfurt 1970; but see
Broughton 2002).


### 1.2 Earlier discussions of dream skepticism and why Descartes' version is special



Dream arguments have been a staple of philosophical skepticism since
antiquity and were so well known that in his objections to the
*Meditations*, Hobbes (1641) criticized Descartes for not
having come up with a more original argument. Yet, Descartes'
version of the problem, more than any other, has left its mark on the
philosophical discussion.



Earlier versions tended to touch upon dreams just briefly and discuss
them alongside other examples of sensory deception. For example, in
the *Theaetetus* (157e), Plato has Socrates discuss a defect in
perception that is common to



> 
> 
> dreams and diseases, including insanity, and everything else that is
> said to cause illusions of sight and hearing and the other senses.
> 
> 
> 
> 



This leads to the conclusion that knowledge cannot be defined through
perception. 



Dreams also appear in the canon of standard skeptical arguments used
by the Pyrrhonists. Again, dreams and sleep are just one of several
conditions (including illness, joy, and sorrow) that cast doubt on the
trusthworthiness of sensory perception (Diogenes Laertius, *Lives
of Eminent Philosophers;* Sextus Empiricus, *Outlines of
Pyrrhonism)*.



Augustine (*Against the Academics*; *Confessions)*
thought the dream problem could be contained, arguing that in
retrospect, we can distinguish both dreams and illusions from actual
perception (Matthew 2005: chapter 8). And Montaigne (*The Apology
for Raymond Sebond*) noted that wakefulness itself teems with
reveries and illusions, which he thought were even more
epistemologically worrisome than nocturnal dreams.



Descartes devoted much more space to the discussion of dreaming and
cast it as a unique epistemological threat distinct from both waking
illusions and evil genius or brain-in-a-vat-style arguments. His claim
that he has often been deceived by his dreams implies he also saw
dreaming as a real-world (rather than merely hypothetical) threat.




This is further highlighted by the intimate, first-person style of the
 *Meditations*. Their narrator is supposed to exemplify
everyone's epistemic situation, illustrating the typical defects of
the human mind. Readers are further drawn in by Descartes' strategy of
moving from commonsense examples towards more sophisticated
philosophical claims (Frankfurt 1970). For example, Descartes builds
up towards dream skepticism by first considering familiar cases of
sensory illusions and then deceptively realistic dreams.



Finally, much attention has been devoted to several dreams Descartes
reportedly had as a young man. Some believe these dreams embodied
theoretical doubts he developed in the *Discourse* and
*Meditations* (Baillet 1691; Leibniz 1880: IV; Cole 1992;
Keefer 1996). Hacking (2001:252) suggests that for Descartes, dream
skepticism was not just a philosophical conundrum but a source of
genuine doubt. There is also some discussion about the dream reports'
authenticity (Freud 1940; Cole 1992; Clarke 2006; Browne 1977).


### 1.3 Dreaming and other skeptical scenarios



In the *Meditations*, after discussing the dream argument,
Descartes raises the possibility of an omnipotent evil genius
determined to deceive us even in our most basic beliefs. Contrary to
dream deception, Descartes emphasizes that the *evil genius
hypothesis* is a mere fiction. Still, it radicalizes the dream
doubt in two respects. One, where the dream argument left the
knowability of certain general truths intact, these are cast in doubt
by the *evil genius hypothesis*. Two, where the dream argument,
at least on the weaker reading, involves just temporary deception, the
evil genius has us permanently deceived.



One modernized version, the *brain-in-a-vat* thought
experiment, says that if evil scientists placed your brain in a vat
and stimulated it just right, your conscious experience would be
exactly the same as if you were still an ordinary, embodied human
being (Putnam 1981). In the *Matrix*-trilogy (Chalmers 2005),
Matrixers live unbeknownst to themselves in a computer simulation.
Unlike the *brain-in-a-vat*, they have bodies that are kept
alive in pods, and flaws in the simulation allow some of them to bend
its rules to their advantage.



Unlike dream deception, which is often cast as a regularly recurring
actuality (cf. Windt 2011), brain-in-a-vat-style arguments are often
thought to be merely logically or nomologically possible. However,
there might be good reasons for thinking that we actually live in a
computer simulation (Bostrom 2003), and if we lend some credence to
radical skeptical scenarios, this may have consequences for how we act
(Schwitzgebel 2017).



Even purely hypothetical skeptical scenarios may enhance their
psychological force by capitalizing on the analogy with dreams. Clark
(2005) argues that the *Matrix* contains elements of
"industrial-strength deception" in which both sensory
experience and intellectual functioning are exactly the same as in
standard wake-states, whereas other aspects are more similar to the
compromised reasoning and bizarre shifts that are the hallmark of
dreams.


### 1.4 Descartes' solution to the dream problem and real-world dreams



At the end of the *Sixth Meditation*, Descartes suggests a
solution to the dream problem that is tied to a reassessment of what
it is like to dream. Contrary to his remarks in the *First
Meditation*, he notes that dreams are only rarely connected to
waking memories and are often discontinuous, as when dream characters
suddenly appear or disappear. He then introduces the coherence
test:



> 
> 
> But when I perceive objects with regard to which I can distinctly
> determine both the place whence they come, and that in which they are,
> and the time at which they appear to me, and when, without
> interruption, I can connect the perception I have of them with the
> whole of the other parts of my life, I am perfectly sure that what I
> thus perceive occurs while I am awake and not during sleep.
> (Meditation VI. 24) 
> 
> 
> 



For all practical purposes, he has now found a mark by which dreaming
and waking can be distinguished (cf. *Meditation* I.7), and
even if the coherence test is not fail-safe, the threat of dream
deception has been averted.



Descartes' remarks about the discontinuous and ad hoc nature of many
dreams are backed up by empirical work on dream bizarreness (see
Hobson 1988; Revonsuo & Salmivalli 1995). Still, many of his
critics were not convinced this helped his case against the skeptic.
Even if Descartes' revised phenomenological description characterizes
most dreams, one might occasionally merely dream of successfully
performing the test (Hobbes 1641), and in some dreams, one might seem
to have a clear and distinct idea but this impression is false
(Bourdin 1641). Both the coherence test and the criterion of clarity
and distinctness would then be unreliable.



How considerations of empirical plausibility impact the dream argument
continues to be a matter of debate. Grundmann (2002) appeals to
scientific dream research to introduce an introspective criterion:
when we introspectively notice that we are able to engage in critical
reflection, we have good reason to think that we are awake and not
dreaming. However, this assumes critical reasoning to be uniformly
absent in dreams. If attempts at critical reasoning do occur in dreams
and if they generally tend to be corrupted, the introspective
criterion might again be problematic (Windt 2011, 2015a). There are
also cases in which even after awakening, people mistake what was in
fact a dream for reality (Wamsley et al. 2014). At least in certain
situations and for some people, dream deception might be a genuine
cause of concern (Windt 2015a).


## 2. The ontology of dreams



In what follows, the term "conscious experience" is used
as an umbrella term for the occurrence of sensations, thoughts,
impressions, emotions etc. in dreams (cf. Dennett 1976). These are all
phenomenal states: there is something it is *like* to be in
these states for the subject of experience (cf. Nagel 1974). To ask
about dream experience is to ask *whether it is like something*
to dream *while* dreaming, and whether *what* it is like
is similar to (or relevantly different from) corresponding waking
experiences.


### 2.1 Are dreams experiences?



Cartesian dream skepticism depends on a seemingly innocent background
assumption: that dreams are conscious experiences. If this is
false, then dreams are not deceptive experiences during sleep and we
cannot be deceived, while dreaming, about anything at all. Whether
dreams are experiences is a major question for the ontology of dreams
and closely bound up with dream skepticism.



The most famous argument denying that dreams are experiences was
formulated by Norman Malcolm (1956, 1959). Today, his position is
commonly rejected as implausible. Still, it set the tone for the
analysis of dreaming as a target phenomenon for philosophy of
mind.



For Malcolm, the denial of dream experience followed from the
conceptual analysis of sleep: "if a person is in *any*
state of consciousness it logically follows that he is not sound
asleep" (Malcolm 1956: 21). Following some remarks of
Wittgenstein's (1953: 184; see Chihara 1965 for discussion),
Malcolm claimed



> 
> 
> the concept of dreaming is derived, not from dreaming, but from
> descriptions of dreams, i.e., from the familiar phenomenon that we
> call "telling a dream". (Malcolm 1959:55)
> 
> 
> 



Malcolm argued that retrospective dream reports are the sole criterion
for determining whether a dream occurred and there is no independent
way of verifying dream reports. While first-person, past-tense
psychological statements (such as "I felt afraid") can at
least in principle be verified by independent observations (but see
Canfield 1961; Siegler 1967; Schroder 1997), he argued dream
reports (such as "in my dream, I felt afraid") are
governed by different grammars and merely superficially resemble
waking reports. In particular, he denied dream reports imply the
occurrence of experiences (such as thoughts, feelings, or judgements)
in sleep:



> 
> 
> If a man had certain thoughts and feelings in a dream it no more
> follows that he had those thoughts and feelings while asleep, than it
> follows from his having climbed a mountain in a dream that he climbed
> a mountain while asleep. (Malcolm 1959/1962: 51-52) 
> 
> 
> 



What exactly Malcolm means by "conscious experience" is
unclear. Sometimes he seems to be saying that conscious experience is
conceptually tied to wakefulness (Malcolm 1956); other times he claims
that terms such as mental activity or conscious experience are vague
and it is senseless to apply them to sleep and dreams (Malcolm 1959:
52).



Malcolm's analysis of dreaming has been criticized as assuming
an overly strict form of verificationism and a naive view of
language and conceptual change. A particularly counterintuitive
consequence of his view is that there can be no observational evidence
for the occurrence of dreams in sleep aside from dream reports. This
includes behavioral evidence such as sleepwalking or sleeptalking,
which he thought showed the person was partially awake; as he also
thought dreams occur in sound sleep, such sleep behaviors were largely
irrelevant to the investigation of dreaming proper. He also claimed
adopting a physiological criterion of dreaming (such as EEG measures
of brain activity during sleep) would change the concept of dreaming,
which he argued was tied exclusively to dream reporting. This claim
was particularly radical as it explicitly targeted the discovery of
REM sleep and its association with dreaming (Dement & Kleitman
1957), which is commonly regarded as the beginning of the science of
sleep and dreaming. Malcolm's position was that the very project of a
science of dreaming was misguided.



Contra Malcolm, most assume that justification does not depend on
strict criteria with the help of which the truth of a statement can be
determined with absolute certainty, but "on appeals to the
simplicity, plausibility, and predictive adequacy of an explanatory
system as a whole" (Chihara & Fodor 1965: 197). In this
view, behavioral and/or physiological evidence can be used to verify
dream reports (Ayer 1960) and the alleged principled difference
between dream reports and other first-person, past-tense psychological
sentences (Siegler 1967; Schroder 1997) disappears.



Putnam noted that Malcolm's analysis of the concept of dreaming
relies on the dubious idea that philosophers have access to deep
conceptual truths that are hidden to laypeople:



> 
> 
> the lexicographer would undoubtedly perceive the logical (or
> semantical) connection between being a pediatrician and being a
> doctor, but he would miss the *allegedly* "logical"
> character of the connection between dreams and waking impressions.
> [...] this "depth grammar" kind of analyticity (or
> "logical dependence") does not exist. (Putnam 1962 [1986]:
> 306) 
> 
> 
> 



Nagel argued that even if one accepts Malcolm's analysis of the
concept of dreaming,



> 
> 
> it is a mistake to invest the demonstration that it is impossible to
> have experiences while asleep with more import than it has. It is an
> observation about our use of the word "experience", and no
> more. It does not imply that nothing goes on in our minds while we
> dream. (Nagel 1959: 114) 
> 
> 
> 



Whether dream thoughts, feelings or beliefs should count as real
instances of their kind now becomes an open question, and in any case
there is no conceptual contradiction involved in saying one has
experiences while asleep and dreaming.


### 2.2 Dreams as instantaneous memory insertions



To ask about dream experience is also to ask whether there is
something it is like to dream during sleep as opposed to there just
being something it is like to remember dreaming after awakening.
Dennett's (1976, 1979) *cassette theory* says dreams are
the product of instantaneous memory insertion at the moment of the
awakening, as if a cassette with pre-scripted dreams had been inserted
into memory, ready for replay. Dennett claims the cassette theory and
the view that dreams are experiences can deal equally well with
empirical evidence for instance on the relationship between dreaming
and REM sleep. The cassette theory is preferable because it is more
parsimonious, positing only an unconscious dream composition process
rather than an additional conscious presentation process in sleep. For
Dennett, the important point is that it is impossible to distinguish
between the two rival theories based on dream recall; the question of
dream experience should be settled by independent empirical
evidence.



While Dennett shares Malcolm's skepticism about dream experience, this
latter claim is diametrically opposed to Malcolm's rejection of
a science of dreaming. For Dennett, the unreliability of dream recall
also is not unique, but exemplifies a broader problem with memory
reports: we generally cannot use retrospective recall to distinguish
conscious experience from memory insertion (Dennett 1991; see also
Emmett 1978).



An earlier and much discussed (Binz 1878; Goblot 1896; Freud 1899;
Hall 1981; Kramer 2007:22-24) version of Dennett's
cassette theory goes back to Maury's (1861) description of a long and
complex dream about the French revolution that culminated in his
execution at the guillotine, at which point Maury suddenly awoke to
find that the headboard had fallen on his neck. Because the dream
seemed to systematically build up to this dramatic conclusion, which
in turn coincided with a sudden external event, he suggested that such
cases were best explained as instantaneous memory insertions
experienced at the moment of awakening. Similarly, Gregory (1916)
described dreams are psychical explosions occurring at the moment of
awakening.



The trustworthiness of dream reports continues to be contentious.
Rosen (2013) argues that dream reports are often fabricated and fail
to accurately describe experiences occurring during sleep. By
contrast, Windt (2013, 2015a) argues that dream reports can at least
under certain conditions (such as in laboratory studies, when dreams
are reported immediately after awakening by trained participants) be
regarded as trustworthy sources of evidence with respect to previous
experience during sleep.


### 2.3 Empirical evidence on the question of dream experience



Unlike Malcolm, many believe that whether dreams are experiences is an
empirical question; and unlike Dennett, the predominant view is that
the empirical evidence does indeed support this claim
(Flanagan 2000; Metzinger 2003; Revonsuo 2006; Rosen 2013; Windt 2013,
2015a).



A first reason for thinking that dreams are experiences during sleep
is the relationship between dreaming and REM (rapid eye movement)
sleep. Researchers in the 1950s discovered that sleep is not a uniform
state of rest and passivity, but there is a sleep architecture
involving different stages of sleep that is relatively stable both
within and across individuals (Aserinsky & Kleitman 1953, 1955;
Dement & Kleitman 1957). Following sleep onset, periods of non-REM
(or NREM) sleep including slow wave sleep (so called because of the
presence of characteristic slow-wave, high-voltage EEG activity) are
followed by periods of high-frequency, low-voltage activity during REM
sleep. EEG measures from REM sleep strongly resemble waking EEG. REM
sleep is additionally characterized by rapid eye movements and a
near-complete loss of muscle tone (Dement 1999: 27-50; Jouvet
1999).



The alignment between conscious experience on the one hand and
wake-like brain activity and muscular paralysis on the other hand
would seem to support the experiential status of dreams as well as
explain the outward passivity that typically accompanies them. Reports
of dreaming are in fact much more frequent following REM (81.9%) than
NREM sleep awakenings (43%; Nielsen 2000). REM reports tend to be more
elaborate, vivid, and emotionally intense, whereas NREM reports tend
to be more thought-like, confused, non-progressive, and repetitive
(Hobson et al. 2000). These differences led to the idea that REM sleep
is an objective marker of dreaming (Dement & Kleitman 1957; Hobson
1988: 154).



Attempts to identify dreaming with mental activity during REM sleep
have not, however, been successful, and many now hold that dreams can
occur in all stages of sleep (e.g., Antrobus 1990; Foulkes 1993b;
Solms 1997, 2000; Domhoff 2003; Nemeth & Fazekas 2018). In recent
years there has been renewed interest in NREM sleep for the study of
dreaming (Noreika et al. 2009; Siclari et al. 2013, 2017). This
suggests the inference from the physiology of REM sleep to the
phenomenology of dreaming is not straightforward.



A second line of evidence comes from lucid dreams, or dreams in which
one knows one is dreaming and often has some level of dream control
(Voss et al. 2013; Voss & Hobson 2015; Baird et al. 2019). The
term lucid dreaming was coined by van Eeden (1913), but Aristotle
(*On Dreams*) already noted that one can sometimes be aware
while dreaming that one is dreaming.



Scientific evidence that lucid dreaming is real and a genuine sleep
phenomenon comes from laboratory studies (Hearne 1978; LaBerge et al.
1981) showing lucid dreamers can use specific, pre-arranged patterns
of eye movements (e.g., right-left-right-left) to signal in real-time
that they are now lucid and engaging in dream experiments. These
signals are clearly identifiable on the EOG and suggest a
correspondence between dream-eye movements and real-eye movements (as
predicted by the so-called *scanning hypothesis*; see Dement
& Kleitman 1957; Leclair-Visonneau et al. 2010). Retrospective
reports confirm that the dreamer really was lucid and signalled
lucidity (Dresler et al. 2012; Stumbrys et al. 2014).



Signal-verified lucid dreams have been used to study muscular activity
accompanying body movements in dreams (Erlacher et al. 2003; Dresler
et al. 2011), for advanced EEG analysis of brain activity during lucid
dreaming (Voss et al. 2009), and imaging studies (Dresler et al. 2011,
2012). Eye signals can also be used to measure the duration of
different activities performed in lucid dreams; contrary to the
cassette theory, lucid dreams have temporal extension and certain
dream actions even seem to take slightly longer than in waking
(Erlacher et al. 2014). There have also been attempts to induce
lucidity through non-invasive electrical stimulation during sleep
(Stumbrys et al. 2013; Voss et al. 2014). The combination of
signal-verified lucid dreaming with volitional control over dream
content, retrospective report, and objective sleep measures has been
proposed to provide controlled conditions for the study of conscious
experience in sleep and a new methodology for investigating the
relationship between conscious experience and neurophysiological
processes (Baird et al.2019).



A third line of evidence (Revonsuo 2006: 77) comes from
dream-enactment behavior (Nielsen et al. 2009), most prominently in
patients with REM-sleep behavior disorder (RBD; Schenck & Mahowald
1996; Schenck 2005; Leclair-Visonneau et al. 2010). Due to a loss of
the muscular atonia that accompanies REM sleep in healthy subjects,
these patients show complex, seemingly goal-directed outward behaviors
such as running or fighting off an attacker during REM sleep.
Retrospective dream reports often match these behaviors, suggesting
that patients literally act out their dreams during sleep.



While persuasive, these lines of evidence might not satisfy skeptics
about dream experience. They might worry that results from lucid
dreaming and dream enactment do not generalize to ordinary, non-lucid
dreams; they might also construe alternative explanations that do not
require conscious experience in sleep. There are also methodological
concerns, for instance about how closely sleep-behaviors actually
match dream experience. A key issue is that to support the
experiential status of dreams, evidence from sleep polysomnography,
signal verified lucid dreams, or sleep behavior requires convergence
with retrospective dream reports. This means trusting dream reports is
built into any attempt to empirically resolve the question of dream
experience - which then invites the familiar skeptical concerns.
Again, an anti-skeptical strategy may be to appeal to explanatory
considerations. In this view, the convergence of dream reports and
objective polysomnographic or behavioral observations is best
explained by the assumption that dreams are experiences in sleep, and
this assumption is strengthened by further incoming findings. This
strategy places dream reports at the center of scientific dream
research while avoiding the contentious claim that their
trustworthiness, and with it the experiential status of dreams, can be
demonstrated conclusively by independent empirical means (Windt 2013,
2015a). 


### 2.4 Dreams and hallucinations



Even where philosophers agree dreams are experiences, they often
disagree on how exactly to characterize dreaming relative to
wake-state psychological terms. Often, questions about the ontology of
dreaming intersect with epistemological issues. Increasingly, they
also incorporate empirical findings.



The standard view is that dreams have the same phenomenal character as
waking perception in that they seemingly put us in contact with
mind-independent objects, yet no such object is actually being
perceived. This means dreams count as hallucinations in the
philosophical sense (Crane & French 2017; Macpherson 2013). Even
if, in a particularly realistic dream, my visual experience was
exactly as it would be if I were awake (I could see my bedroom, my
hands on the bed sheets, etc.), as long as my eyes were closed during
the episode, I would not, literally, be *seeing* anything.



There is some controversy in the psychological literature about
whether dreams should be regarded as hallucinations. Some believe the
term hallucination should be reserved for clinical contexts and
wake-state pathologies (Aleman & Laroi 2008: 17; but see
ffytche 2007; ffytche et al. 2010).



The view that dreams involve hallucinations is implicit in
Descartes' assumption that even when dreaming,



> 
> 
> it is certain that I seem to see light, hear a noise, and feel heat;
> this cannot be false, and this is what in me is properly called
> perceiving (*sentire*). (Descartes 1641: II.9) 
> 
> 
> 



It also lies at the heart of Aristotle's (*On Dreams*)
assumption that dreams result from the movements of the sensory organs
that continue even after the original stimulus has ceased. He believed
that in the silence of sleep, these residual movements result in vivid
sensory imagery that is subjectively indistinguishable from genuine
perception (see also Dreisbach 2000; Barbera 2008).



The assumption of phenomenological equivalence between dream and
waking experience can also be found in Berkeley's (1710: I.18)
idealist claim that the existence of external bodies is not necessary
for the production of vivid, wake-like perceptual experience.
Similarly, Russell defended sense-data theory by noting that in
dreams, 



> 
> 
> I have all the experiences that I seem to have; it is only things
> outside my mind that are not as I believe them to be while I am
> dreaming. (Russell 1948: 149-150) 
> 
> 
> 



Elsewhere, he argued dreams and waking life



> 
> 
> must be treated with equal respect; it is only by some reality not
> *merely* sensible that dreams can be condemned. (Russell 1914:
> 69) 
> 
> 
> 



Hume was less clear on this matter, proposing that dreams occupy an
intermediate position between vivid and largely non-voluntary sensory
impressions and ideas, or "the faint images of previous
impressions in thinking and reasoning" (Hume 1739: 1.1.1.1). On
the one hand, as mere creatures of the mind, Hume wanted to categorize
dreams as ideas. On the other hand, he acknowledged that in sleep,
"our ideas can approach the vivacity of sensory
impressions" (Hume 1739: 1.1.1.1). Dreams do not fit comfortably
into Hume's attempt to draw a dichotomous distinction between
impressions, including perception, and ideas, including sensory
imagination (Ryle 1949; Waxman 1994; Broughton 2006).



Phenomenologists often focus not so much on the quality of dream
imagery as on the overall character of experience, noting that dreams
are experienced as reality; as in waking perception, we simply feel
present in a world. This also sets dreams apart from waking fantasy
and daydreams (Husserl 1904/1905; Uslar 1964; Conrad 1968; Globus
1987: 89. 



At its strongest, the hallucination view claims that dreaming and
waking experience are identical in both the quality of sensory imagery
and their overall, self-in-a-world structure (Revonsuo 2006: 84). This
claim is central to the *virtual reality metaphor*, according
to which consciousness itself is dreamlike and waking perception a
kind of online hallucination modulated by the senses (Llinas
& Ribary 1994; Llinas & Pare 1991; Revonsuo
2006; Metzinger 2003, 2009).



This seems to be empirically supported. Neuroimaging studies (Dang-Vu
et al. 2007; Nir & Tononi 2010; Desseilles et al. 2011) show that
the predominance of visual and motor imagery as well as strong
emotions in dreams is paralleled by high activation of the
corresponding brain areas in REM sleep, which may exceed waking; at
the same time, the cognitive deficits often thought to characterize
dreams such as the loss of self-awareness, the absence of critical
thinking, delusional reasoning, and mnemonic deficits fit in well with
the comparative deactivation of frontal areas (Hobson et al. 2000).
Hobson (1988, Hobson et al. 2000) has argued that the vivid,
hallucinatory character of dreaming results from the fact that in REM
sleep, the visual and motor areas are activated in the same way as in
waking perception, the sole difference being dreams' dependence on
internal signal generation. Horikawa and colleagues (2013) used
neuroimaging data from sleep onset to predict the types of objects
described in mentation reports, which they took to support the
perceptual equivalence between dreaming and waking. 



Generally, versions of the hallucination view that suggest dreams
replicate all aspects of waking perception are too vague to be
informative. Especially for subtle perceptual activities (such as
visual search), we might not know enough about dream phenomenology to
make any strong claims (Nielsen 2010). Specifying points of similarity
leads to a more informative and precise, but likely also more nuanced
view. Dreams are heterogeneous, and some might be more perception-like
while others resemble imagination (Windt 2015a). There might also be
differences between or even within specific types of imagery. For
example, visual imagery might be quite different from touch
sensations, which tend to be rare in dreams (Hobson 1988). Visual
dream imagery might overall resemble waking perception but lack color
saturation, background detail and focus (Rechtschaffen &
Buchignani, 1992). Classifying dreams as either hallucinatory or
imaginative is further complicated by the fact that there is strong
overlap in cortical activity associated with both visual imagery and
perception (Zeidman & Maguire, 2016). This means even a strong
overlap in cortical activity between, say, visual dream imagery and
visual perception does not necessarily set dreaming apart from waking
imagination.



This is also true for evidence on eye movements in dreams. LaBerge and
colleagues (2018) recently showed that eye tracking of objects is
smooth in lucid dreaming and perceiving, but not in imagining. Drawing
from this evidence, Rosen (forthcoming) suggests many dreams mimic the
phenomenology of interacting with a stable world, including eye
movements and visual search. Others argue we should not analogize
dream imagery to mind-independent, scannable objects and that eye
movements might instead be implicated in the generation of dream
imagery (Windt 2018).


### 2.5 Dreams and illusions



Another way to make sense of the claim that dreaming has the same
phenomenal character as waking perception is to say some kinds of
dream imagery are illusory: they involve misperception of an external
object as having different properties than it actually has (cf. Smith
2002; Crane & French 2017). The illusion view disagrees with the
hallucination view on whether dreams have a contemporaneous external
stimulus source.



The illusion view has fallen out of favor but has a long history. The
Ancients believed dreams have bodily sources. This idea underlies the
practice of using dreams to diagnose illness, as practiced in the
shrines at Epidaurus (Galen *On Diagnosis in Dreams*; van de
Castle 1994). Aristotle (*On Dreams*) thought some dreams are
caused by indigestion, and Hobbes adopted this view, claiming
different kinds of dreams could be traced to different bodily
sensations. For instance, "lying cold breedeth Dreams of Feare,
and raiseth the thought and Image of some fearfull object"
(Hobbes 1651: 91).



Appeals to the bodily sources of dreaming became especially popular in
the 19th and early 20th centuries. Many believed
specific dream themes such as flying were linked to sleeping position
(Macnish 1838; Scherner 1861; Vold 1910/1912; Ellis 1911) and
realizing, in sleep, that one's feet are not touching the ground
(Bergson 1914).



There were also attempts to explain the phenomenology of dreaming by
appealing to the absence of outward movement. The lack of appropriate
feedback and of movement and touch sensations was thought to cause
dreams of being unable to move (Bradley 1894) or of trying but failing
to do something (Gregory 1918).



Some proponents of the "*Leibreiztheorie*" (or
somatic-stimulus theory) of dreaming attempted to go beyond anecdotal
observations to conduct controlled experiments. Weygandt (1893)
investigated the influence of various factors including breathing,
blood circulation, temperature changes, urge to urinate, sleeping
position, and visual or auditory stimulation during sleep on dream
content (see Schredl 2010 for details). Singer (1924) proposed
experiments on stimulus incorporation in dreams can inform claims on
the ontology of dreaming: If dreams are sensations, a particular
auditory stimulus should increase the frequency of dreams in nearby
sleepers as well as the frequency of sound in their dreams, and it
should decrease the range of quality and intensity of these dreams,
making them overall more similar and predictable.



Newer studies provide evidence for the incorporation of external
stimuli in dreams, including light flashes, sounds, sprays of water
applied to the skin (Dement & Wolpert 1958), thermal (Baldridge
1966), electrical (Koulack 1969), and verbal stimuli (Berger 1963;
Breger et al. 1971; Hoelscher et al., 1981), as well as blood pressure
cuff stimulation on the leg (Nielsen et al. 1995; Sauvageau et al.
1998).



Muscular activity also often leaves its mark on dreams. It occurs
throughout sleep but is especially frequent in REM sleep, mostly in
the form of twitching but occasionally also in the form of larger,
seemingly goal-directed movements (Blumberg 2010; Blumberg &
Plumeau 2016). The relation between outward and dream movements is
complex: in some cases, outward movements might mirror dream
movements, while in others, sensory feedback might prompt dream
imagery (Windt 2018).



Generally, it seems external and bodily stimuli can be related to
varying degrees to dream and sleep onset imagery (Nielsen 2017; Windt
2018; Windt et al. 2016). Some of these cases appear to fit the
concept of illusion, as in when the sound of the alarm clock is
experienced, in a dream, as a siren, or when blood pressure cuff
inflation on the leg leads to dreams of wearing strange shoes (Windt
2018; for these and other examples, see Nielsen et al. 1995). In other
cases, such as when blood pressure cuff stimulation on the leg prompts
a dream of seeing someone else's leg being run over, describing this
as illusory misperception might be less straightforward.



Saying that dreams can be prompted by external stimuli and that in
some cases these are best described as illusions is different from the
stronger claim, sometimes advanced by historical proponents of
somatic-stimulus theory, that dreams *generally* are caused by
external or bodily stimuli. As an example of the stronger claim,
consider Wundt's proposal that the



> 
> 
> ideas which arise in dreams come, at least to a great extent, from
> sensations, especially from those of the general sense, and are
> therefore mostly illusions of fancy, probably only seldom pure memory
> ideas which hence become hallucinations. (Wundt 1896: 179) 
> 
> 
> 



This claim is likely too strong. It is also likely that appeals to
external or bodily stimuli on their own cannot fully explain dream
imagery, including when and how external stimuli are incorporated in
dreams. Sensory incorporation in dreams is often hard to predict and
indirect; associated imagery seems related not just to stimulus
intensity, but also to short- and long term memories. A full
explanation of dream content additionally has to take the cognitive
and memory sources of dreaming into account (Windt 2018; Nielsen 2017;
cf. Silberer 1919).


### 2.6 Dreams as imaginative experiences



The most important rival to the hallucination view is that dreams are
imaginative experiences (Liao & Gendler 2019; Thomas 2014). This
can mean dream imagery involves imaginings rather than percepts
(including hallucinations or illusions; McGinn 2004), that dream
beliefs are imaginative and not real beliefs (Sosa 2007), or both
(Ichikawa 2008, 2009). An important advantage is that by assimilating
dreams to commonplace mental states such as waking fantasy and
daydreaming, rather than a rare and often pathological occurrence such
as hallucinations, it provides a more unified account of mental life
(Stone 1984). However, the reasons for adopting the imagination view
are diverse, and dreams have been proposed to resemble imaginings and
differ from perception along a number of dimensions (e.g. McGinn 2004,
2005a,b; Thomas 2014). This issue is complicated by the fact that
there is little agreement on the definition of imagination and its
relation to perception (Kind 2013).



One way is to deny dreams involve presence or the feeling of being in
a world, which many believe is central to waking perception.
Imagination theorists compare the sense in which we feel present in
our dreams to cognitive absorption, as when we are lost in a novel,
film, or vivid daydream (Sartre 1940; McGinn 2004; but see Hering
1947; Globus 1987). Some argue that reflexive consciousness or
meta-awareness (as in lucid dreams) interrupts cognitive absorption
and terminates the ongoing dream (Sartre 1940), essentially denying
lucid dreams are possible.



Another issue is whether dreams are *subject to the will*
(Ichikawa 2009). Imagination is often characterized as active and
under our control (Wittgenstein 1967: 621, 633), involving "a
special effort of the mind" (Descartes 1641: VI, 2), whereas
perception is passive. Because dreams just seem to happen to us
without being under voluntary control, they present an important
challenge for the imagination view. Ichikawa (2009) argues lucid
control dreams show dreams are generally subject to the will even
where they are not under deliberate control.



Dreams are widely described as more indeterminate than waking
perception (James 1890: 47; Stone 1984). In scientific dream research,
vagueness is regarded as one of three main subtypes of bizarreness
(Hobson 1988; Revonsuo & Salmivalli 1995). An example are dream
characters who are identified not by their behavior or looks, but by
*just knowing* (Kahn et al. 2000, 2002; Revonsuo & Tarkko
2002). Dreams are also attention-dependent and lack
foreground-background structure (Thompson 2014); while it is tempting
to construe the dream world as rich in detail, there is no more to
dreams than meets the eye, and many think dream experience is
exhausted by what is the focus of selective attention (Hunter 1983;
Thompson 2014).



Indeterminacy is also related to the question of whether we dream in
color or in black and white. Based on a review of historical and
recent studies, Schwitzgebel (2002, 2011) argues there has been a
shift in theories on dream color that coincides with the rise first of
black-and-white and then color television. He argues it is unlikely
that dreams themselves changed from colored to black and white and
back to colored, proposing that a change in opinion is a more
plausible explanation. Maybe dreams were either black and white or
colored all along; or maybe they are indeterminate with respect to
color, as may be the case for imagined or fictional objects; were this
the case, it would strengthen the imagination view (Ichikawa 2009).
Schwitzgebel's main point is that reports of colored dreaming are
unreliable and our opinions about dreams can be mistaken (but see
Windt 2013, 2015a). This relates to Schwitzgebel's (2011; Hurlburt
& Schwitzgebel 2007) general skepticism about the reliability of
introspection.



The issue of dream color has led to a number of follow-up studies
(Schwitzgebel 2003; Schwitzgebel et al. 2006; Murzyn 2008; Schredl et
al. 2008; Hoss 2010). They suggest most people dream in color and a
small percentage describe grayscale or even mixed dreams (Murzyn 2008)
or dreams involving moderate color saturation (Rechtschaffen and
Buchignani 1992). Indeterminacy is rarely reported.



The imagination view has consequences for Cartesian dream skepticism.
If dream pain does not feel like real pain, there is a fail-safe way
to determine whether one is now dreaming: one need only pinch oneself
(Nelson 1966; Stone 1984; but see Hodges & Carter 1969; Kantor
1970). As Locke put it,



> 
> 
> if our dreamer pleases to try, whether the glowing heat of a glass
> furnace, be barely a wandering imagination in a drowsy man's
> fancy, by putting his hand into it, he may perhaps be wakened into a
> certainty greater than he could wish, that it is something more than
> bare imagination. (Locke 1689: IV.XI.8) 
> 
> 
> 



If dreaming feels different from waking, this raises the question why
we tend to describe dreams in the same terms as waking perception.
Maybe this is because most people haven't thought about these matters
and they would find the imagination view plausible if they considered
it (Ichikawa 2009). Or maybe



> 
> 
> it is just because we all know that dreams are *throughout
> un*like waking experiences that we can safely use ordinary
> expressions in the narration of them. (Austin 1962: 42) 
> 
> 
> 



Some authors classify dreams as imaginings while acknowledging they
feel like perceiving. For example, Hobbes describes dreams as
"the imaginations of them that sleep" (Hobbes 1651: 90),
and imagination as a "*decaying sense*" (Hobbes
1651: 88). Yet he also uses the concepts of imagination and fancy to
describe perception and argues "their appearance to us is Fancy,
the same waking, that dreaming" (Hobbes 1651: 86).



In the scientific literature, the imagination view is complemented by
cognitive theories. Foulkes (1978: 5) describes dreaming as a form of
thinking with its own grammar and syntax, but allows that dream
imagery is sufficently perception-like to deceive us. Domhoff's
*neurocognitive model of dreaming* (2001, 2003) emphasizes the
dependence of dreaming on visuospatial skills and on a network
including the association areas of the forebrain. The theory draws
from findings on the partial or global cessation of dreaming following
brain lesions (cf. Solms 1997, 2000), evidence that dreaming develops
gradually and in tandem with visuospatial skills in children (Foulkes
1993a, 1999; but see Resnick et al. 1994), and results from dream
content analysis supporting the continuity of dreaming with waking
concerns and memories (the so-called *continuity hypothesis*;
see Domhoff 2001, 2003; Schredl & Hofmann 2003; Schredl 2006; see
also Nir & Tononi 2010).


### 2.7 Dreaming and waking mind wandering



A number of researchers have begun to consider dreaming in the context
of theories of mind wandering. Mind wandering is frequent in waking
and involves spontaneous thoughts that unfold dynamically and are only
weakly constrained by ongoing tasks and environmental demands
(Schooler et al. 2011; Smallwood & Schooler 2015; Christoff et al.
2016). Based on phenomenological and neurophysiological similarities,
dreams have been proposed to be an intensified form of waking mind
wandering (Pace-Schott 2007, 2013; Domhoff 2011; Wamsley 2013; Fox et
al. 2013). This basic idea seems to have been anticipated by Leibniz,
who noted that the spontaneous formation of visions in dreams
surpasses the capacity of our waking imagination (Leibniz,
*Philosophical Papers and Letters*, Vol. I, 177-178).



The analogy between dreams and waking mind wandering has been
discussed in the context of cognitive agency. Metzinger (2013a,b,
2015) describes dreams and waking mind wandering as involving a
cyclically recurring loss of mental autonomy, or the ability to
deliberately control one's conscious thought processes. Dreams
and waking mind wandering are not mental actions but unintentional
mental behaviors, comparable to subpersonal processes such as
breathing or heartbeat. Because dreaming and waking mind wandering
make up a the majority of our conscious mental lives, he argues that
cognitive agency and mental autonomy are the exception, not the
rule.



This raises the question of how to make sense of lucid control dreams,
which involve both meta-awareness and agency. Windt and Voss (2018)
argue that in such cases, spontaneous processes including imagery
formation co-exist alongside more deliberate, top-down control; they
also argue metacognitive insight and control themselves can have
spontaneous elements. This suggests spontaneity and control are not
opposites, but a more complex account is needed. Possibly, certain
dreams and instances of waking mind wandering can be both spontaneous
and agentive.



The analogy with mind wandering might help move forward the debate on
the ontology of dreaming. In this debate, a common assumption is that
dreams can be categorized as either hallucinatory or imaginative. Yet
the application of these terms to dreams quickly runs into
counterexamples and it is unclear they are mutually exclusive. One
option is pluralism (Rosen 2018b), in which some aspects of dreaming
are hallucinatory, others imaginative, and yet again others illusory.
Another is that dreams are sui generis, combining aspects associated
with wake states such as hallucinating, imagining, or perceiving in a
novel manner without mimicking them completely. Windt (2015a) proposes
that mind wandering, which describes a range of mental states loosely
characterized by their spontaneous and dynamic character, might be
particularly suitable for the characterization of dreaming precisely
because that term leaves open more specific questions on the
phenomenology of dreaming, allowing for variation in control,
determinacy, and so on. This might be a good starting point for
describing what is unique about dreaming while also acknowledging
continuities across sleep-wake states and capitalizing on the
strengths of the hallucination, illusion, imagination, and cognitive
views.


### 2.8 The problem of dream belief



The second strand of the imagination view argues that dream beliefs
are not real beliefs, but propositional imaginings. This may or may
not be combined with the claim that dream imagery is imaginative
rather than perceptual (Sosa 2007; Ichikawa 2009).



Denying that dream beliefs have the status of real beliefs only makes
sense before the background of a specific account of what beliefs are
and how they are distinguished from other mental states such as
delusions or propositional imaginings. For instance, Ichikawa (2009)
argues that if we follow interpretationist or dispositionalist
accounts of belief, dream beliefs fall short of real beliefs. He
claims dream beliefs lack connection with perceptual experience and
fail to motivate actions; consequently, they do not have the same
functional role as real beliefs. Moreover, we cannot ascribe dream
beliefs to a person by observing them lying asleep in bed. Dream
beliefs are often inconsistent with longstanding waking beliefs and
acquired and discarded without any process of belief revision
(Ichikawa 2009).



This analysis of dream beliefs has consequences for skepticism. If
dream beliefs are propositional imaginings, then we do not falsely
believe while dreaming that we are now awake, but only imagine that we
do (Sosa 2007). It is not clear though that this protects us from
deception. If dream beliefs fall short of real beliefs, this might
even make the specter of dream deception more worrisome: in mistaking
dream beliefs for the real thing, we would now be deceived about the
status of our own mental states (Ichikawa 2008).



It is also not clear whether the same type of argument extends to
mental states other than beliefs. As Lewis points out, a person
might



> 
> 
> in fact believe or realize in the course of a dream that he was
> dreaming, and even if we said that, in such case, he only dreamt that
> he was dreaming, this still leaves it possible for someone who is
> asleep to entertain at the time the thought that he is asleep. (Lewis
> 1969: 133) 
> 
> 
> 



Mental states other than believing such as entertaining, thinking, or
minimally appraisive instances of taking for granted might be
sufficient for deception (Reed 1979).



The debate about dream beliefs is paralleled by a debate about whether
delusions are beliefs or imaginings (see Currie 2000; Currie &
Ravenscroft 2002; McGinn 2004; Bayne & Pacherie 2005; Bortolotti
2009; Gendler 2013). Both debates might plausibly inform each other,
especially as dreams are sometimes proposed to be delusional (Hobson
1999).


## 3. Dreaming and theories of consciousness


### 3.1 Dreaming as a model system and test case for consciousness research



Dreams are a global state of consciousness in which experience arises
under altered behavioral and neurophysiological conditions as compared
to standard wakefulness; unlike other altered states of consciousness
(such as drug-induced or deep meditative states) and pathological wake
states (such as psychosis or neurological syndromes), dreams occur
spontaneously and regularly in healthy subjects. For both reasons,
many regard dreams as a test case for theories of consciousness or
even an ideal model system for consciousness research (Churchland 1988;
Revonsuo 2006).



Existing proposals differ on the phenomenology of dreaming: referring
to dream bizarreness, Churchland describes dream experience as
robustly different from waking, whereas Revonsuo argues dreaming is
similar to waking and the purest form of experience:



> 
> 
> the dreaming brain brings out the phenomenal level of organization in
> a clear and distinct form. Dreaming is phenomenality pure and simple,
> untouched by external physical stimulation or behavioural activity.
> (Revonsuo 2006: 75) 
> 
> 
> 



Revonsuo argues dreaming reveals the basic, state-independent
structure of consciousness to be immersive: "dreaming depicts
consciousness first and foremost as a subjective
world-*for-me*" (Revonsuo 2006: 75). This leads him to
introduce the "world-simulation metaphor of
consciousness", according to which consciousness itself is
essentially simulational and dreamlike. This is taken to support
internalism about conscious experience.



This latter claim is also contentious. Noe (2004: 213) argues
that phenomenological differences between dreaming and waking (such as
greater instability of visual dream imagery) result from the lack of
dynamic interaction with the environment in dreams. He proposes this
shows that neural states are sufficient for dreaming but denies they
are also sufficient for perceptual experience.



A possible problem for both views is their reliance on background
assumptions about the phenomenology of dreaming and its disconnection
from environmental stimuli and bodily sensations. Windt (2015a, 2018)
argues both internalism and externalism mistakenly assume dreams to be
isolated from external sensory input and own-body perception; she
believes both the phenomenology of dreaming and its correlation with
external stimuli are complex and variable. She argues the analysis of
dreaming does not clearly support either side in the debate on
internalism vs externalism (but see Rosen 2018a). Generally, in the
absence of a well worked out theory of dreaming and its sleep-stage
and neural correlates, proposals for using dreaming as a model system
or test case run the risk of relying on an oversimplified description
of the target phenomenon (Windt & Noreika 2011).



Recent accounts appealing to generative models and predictive
processing (Clark 2013b; Hohwy 2013) suggest a new, unified account of
perception, imagination, and dreaming. In these accounts, different
mental states, including perception and action, embody different
strategies of hypothesis testing and prediction error minimization.
Perception is the attempt to model the hidden external causes of
sensory stimuli; action involves keeping the internal model stable
while changing the sensory input. Clark argues that on such a
model,



> 
> 
> systems that know how to perceive an object as a cat are thus systems
> that, *ipso facto*, are able to use a top-down cascade to bring
> about the kinds of activity pattern that would be characteristic of
> the presence of a cat. [...] Perceivers like us, if this is
> correct, are inevitably potential dreamers and imaginers too.
> Moreover, they are beings who, in dreaming and imagining, are
> deploying many of the very same strategies and resources used in
> ordinary perception. (Clark 2013a: 764) 
> 
> 
> 



Predictive processing accounts have also been used to explain specific
features of dreaming. Bizarreness has been associated with the
comparative lack of external stimulus processing, implying dream
imagery is relatively unconstrained by prediction errors (cf. Hobson
& Friston 2012; Fletcher & Frith 2008; Bucci & Grasso
2017). Windt (2018) suggests a predictive processing account of dream
imagery generation that links bodily self-experience to own-body
perception and subtle motor behaviors such as twitching in REM sleep
(Blumberg 2010; Blumberg & Plumeau 2016). She argues that movement
sensations in dreams, in relation to REM-sleep related muscle
twitching, involve a form of bodily self-sampling in which coordinated
muscular activity contributes to the generation and maintenance of a
body model. This is important because in predictive processing
accounts neither the bodily nor the external causes of sensory inputs
are known; at the same time, having an accurate body model is a
prerequisite for action, requiring the system to disambiguate between
self- and other generated changes to sensory inputs. Especially in
early development, sleep might provide the ideal conditions for
exploring one's own body via subtle but coordinated muscular activity
while processing of visual and auditory stimuli is reduced.



Dreams have also been suggested as a test case for whether phenomenal
consciousness can be divorced from cognitive access (e.g., Block 2007;
but see Cohen & Dennett 2011). Sebastian (2014a) argues
that dreams provide empirical evidence that conscious experience can
occur independently of cognitive access. This is because during
(non-lucid) REM-sleep dreams, the dorsolateral prefrontal cortex
(dlPFC) as the most plausible mechanism underlying cognitive access is
selectively deactivated (see also Pantani et al. 2018). This would
challenge theories linking conscious experience to access, such as
higher-order-thought theory (Sebastian 2014b). However, both
the hypoactivation of the dlPCF in REM sleep and its association with
cognitive access have been debated. Fazekas and Nemeth (2018) suggest
that certain kinds of cognitive access may be independent of dlPFC
activation, necessitating a more complex account.


### 3.2 Dreams, psychosis, and delusions



Dreaming has been suggested as a model system not just of waking
consciousness in general, but also of psychotic wake states in
particular. The analogy between dreaming and madness has a long
philosophical history (Plato, *Phaedrus*; Kant 1766;
Schopenhauer 1847) and finds particularly stark expression in
Hobson's claim that "dreaming is not a model of a
psychosis. It is a psychosis. It's just a healthy one"
(Hobson 1999: 44). Gottesmann (2006) proposes dreaming as a
neurophysiological model of schizophrenia. There is a rich discussion
on the theoretical and methodological implications of dream research
for psychiatry (see Scarone et al. 2007; d'Agostino et al. 2013;
see Windt & Noreika 2011 as well as the other papers in this
special issue) and a number of studies have investigated differences
in dream reports from schizophrenic and healthy subjects (Limosani et
al. 2011a,b).



Rather than likening dreaming to waking in general or specific wake
states such as psychosis, there have also been attempts to compare
specific dream phenomena to wake-state delusions. Gerrans (2012, 2013,
2014) focuses on character misidentification in dreams and delusions
of hyperfamiliarity (such as the Fregoli delusion, in which
strangers are mistakenly identified as family members, and
*deja vu*) to argue that anomalous experience and
faulty reality testing both play a role in delusion formation. Rosen
(2015) analyzes instances of thought insertion and of auditory
hallucinations, which are key symptoms of schizophrenia, to raise
broader questions about the altered sense of agency in dreams as
compared to waking.


### 3.3 Beyond dreams: Dreamless sleep experience and the concepts of sleep, waking, and consciousness



Philosophers have focused almost exclusively on dreaming, largely
leaving to the side questions about dreamless sleep including whether
it is uniformly unconscious. In recent years there has been a surge of
interest in the possibility of dreamless sleep experience and
foundational issues about the definition of sleep and waking. This has
been paralleled by growing interest in dreaming in NREM sleep.



Conceptually, interest in dreamless sleep experience has been
facilitated by the precise definition of dreaming offered by
simulation views (Revonsuo et al. 2015). If dreams are immersive sleep
experiences characterized by their *here*-and-*now*
structure, it makes sense to ask whether this is true for all or just
a subset of sleep-related experiences and whether non-immersive sleep
experiences exist. By contrast, if dreaming is broadly identified with
any conscious mentation in sleep (Pagel et al. 2001), there is no
conceptual space for dreamless sleep experience.



Following Thompson's (2014, 2015) discussion of dreamless sleep in
Indian and Buddhist philosophy, Windt and colleagues (2016; see also
Windt 2015b) introduce a framework for different kinds of dreamless
sleep experience ranging from thinking and isolated imagery,
perception, or bodily sensations, where these lack integration into a
scene, to minimal kinds of experience lacking imagery or specific
thought contents. A possible example of minimal phenomenal experience
in sleep are white dreams, where people report having had experiences
during sleep but cannot remember any details. Taken at face value,
some white dream reports might describe experiences that lack
reportable content (Windt 2015b); others might describe forgotten
dreams or dreams with degraded content (Fazekas et al. 2018). Another
example are reports of witnessing dreamless sleep, as described in
certain meditation practices. This state is said to involve
non-conceptual awareness of sleep, again in the absence of imagery or
specific thought contents, and loss of sense of self (Thompson 2014,
2015). Some schools in Buddhist philosophy explain claims of deep and
dreamless sleep by saying we never fully lose consciousness in sleep
(Prasad 2000, 66; and Thompson 2014, 2015).



Empirically, interest in dreamless sleep experience is paralleled by
increasing interest in experiences in NREM sleep (Fazekas et al.
2018). Most researchers now accept that dreaming is not confined to
REM sleep, but also occurs at sleep onset and in NREM sleep. The
deeper stages of NREM sleep are particularly interesting as they
involve roughly similar proportions of dreaming, unconscious sleep,
and white dreams (Noreika et al. 2009: Siclari et al. 2013, 2017). In
the search for the neural correlates of dreaming vs unconscious
dreamless sleep, this makes comparisons within the same sleep stage
possible and avoids confounds involved in comparing presumably
dreamful REM sleep with presumably dreamless NREM sleep. Findings
suggest that activity in the same parietal hot zone underlies dreaming
in both NREM and REM sleep (Siclari et al. 2017).



Where sleep and dream research have traditionally tried to identify
the sleep stage correlates of dreaming, newer research suggests local
changes occurring independently of sleep stages might in fact be more
relevant. Traditionally regarded as global, whole-brain phenomena,
there is now increasing evidence that sleep itself is locally driven,
and local changes in sleep depth might be associated with changes in
sleep-related experience (Siclari & Tononi 2017; Andrillon et al.
2019). While sleep and dream research are often considered as separate
fields, changes in how sleep in general and sleep stages in particular
are defined appear closely associated with changes in the theoretical
conception of dreaming and its empirical investigation.



Historically, discoveries about dreaming have precipitated changing
conceptions of sleep (for an excellent history of the study of sleep
and dreaming, see Kroker 2007). Following Aristotle (*On Sleeping
and Waking*), sleep was traditionally defined in negative terms as
the absence of wakefulness and perception. This is still reflected in
Malcolm's assumption that "to a person who is sound
asleep, 'dead to the world', things cannot even
seem" (Malcolm 1956: 26). With the discovery of REM sleep,
sleep came to be regarded as a heterogeneous phenomenon characterized
by the cyclic alteration of different sleep stages. REM sleep was now
considered as "neither sleeping nor waking. It was obviously a
*third state* of the brain, as different from sleep as sleep is
from wakefulness" (Jouvet 1999: 5). The folk-psychological
dichotomy between sleep and wakefulness now seemed oversimplified and
empirically implausible. At the same time dreaming, which had
previously been considered as an intermediate state of half-sleeping
and half-waking, came to be regarded as a genuine sleep phenomenon,
but narrowed to REM sleep. Today, the framework for describing dreams
and other sleep-related experiences is more precise, but dreaming has
also been cast adrift from REM sleep.



A closely associated issue is how to define waking. Crowther's
(2018) capacitation thesis casts waking consciousness as a state in
which the individual is fully switched on to their environment, but
also to their own epistemic (cf. O'Shaugnessy 2002) and agentive
potential; the waking individual is empowered to act and think in
certain ways, though this potential need not be actualized. By
contrast, dreaming is an "imagining-of consciousness"
(O'Shaughnessy 2002: 430) and consciousness is conceptually tied
to wakefulness. Because in lucid dreams, the epistemic and agentive
profile of waking is at least partly realized, they might, according
to Crowther, be regarded as closer to waking than nonlucid dreams.



This account of waking and sleep may also have consequences for the
imagination model of dreaming and dream skepticism (Soteriou 2017). As
in the imagination model, dreaming would be passive and action,
including cognitive agency, would be tied to waking. If dreaming
nonetheless involved passive episodes of imagining oneself to be
active, one would be unable to tell that one were dreaming and
imagining, as this insight would require the exercise of real agency.
The sceptical consequence would be that when dreaming, one would lose
agency as well as the capacity to gain insight into one's
current state. Yet our ability to know we are waking when waking would
be unscathed; according to Soteriou, waking would thus have an
epistemic function connected to the capacity to exercise agency over
our mental lives.



Finally, definitions of consciousness themselves are bound up with
conceptions of sleep and dreaming. As dreaming went from a state whose
experiential status was doubted to being widely recognized as a second
global state of consciousness, consciousness sometimes came to be
defined contrastively as that which disappears in deep, dreamless
sleep and reappears in waking and dreaming (Searle 2000; Tononi 2008).
In light of dreamless sleep experience, such definitions are
problematic (Thompson 2014, 2015; Windt 2015b; Windt et al. 2016).
Dreamless sleep experience has been proposed to be particularly
relevant for understanding minimal phenomenal experience, or the
conditions under which the simplest kind of conscious experience
arises (Windt 2015b). The investigation of dreamless sleep might thus
shed light on the transition from unconscious sleep to sleep-related
experience.


## 4. Dreaming and the self



We almost always have a self in dreams, though this self can sometimes
be a slightly different (e.g. older or younger) version of our waking
self or even a different person entirely. Dreams therefore raise
interesting questions about the identity between the dream and waking
self. Locke (1689) invites us to imagine two men alternating in turns
between sleep and wakefulness and sharing one continuously thinking
soul (Locke 1689: II.I.12). He argues that if one man retained no
memory of the soul's thoughts and perceptions while it was
linked to the other man's body, they would be distinct persons.
His position is that personal identity depends on psychological
continuity, including recall: in the absence of recall, as illustrated
by the toy example of two people sharing one soul, continuous
conscious thinking does not suffice for identity. Locke also rejects
the possibility of unrecalled dreams and the idea that we dream
throughout sleep, remembering only a small proportion of our dreams
(Locke 1689: II.I.19).



Valberg distinguishes between the subject of the dream (i.e., the
dream self) and the sleeping person who is the dreamer of the dream
and recalls it upon awakening (Valberg 2007). He argues that awakening
from a dream involves crossing a chasm between discrete worlds with
discrete spaces and times; it does not make sense to say that
"the 'I' at these times [is] a single individual who
crosses from one world to the other" (Valberg 2007: 69).
According to Valberg, this is relevant to dream skepticism because
there is no simple way to make sense of the claims that it is I who
emerge from a dream or that I was the victim of dream deception.



Vicarious dreams, or dreams in which the protagonist of the dream
seems to be a different person from the dreamer, are particularly
puzzling with respect to identity. They may even raise the question of
whether the dream self has an independent existence (Rosen &
Sutton 2013: 1047). Such dreams are superficially similar to cases in
which we imagine being another person, but according to Rosen and
Sutton require a different explanation: in the case of dreaming, the
imagined person's thoughts are not framed as diverging from
one's own and one does not retain one's own perspective in
addition to the imagined one; in nonlucid dreams, only the perspective
of the dream's protagonist is retained.



The dream self is also at the center of simulation views of dreaming,
which define dreaming via its immersive, here and now character as the
experience of a self in a world. This leads to further questions about
the phenomenology of self-experience in dreams and how it is different
from waking self-experience. Different versions of the simulation view
focus on different aspects of self- and world experience in dreams,
ranging from social simulation (Revonsuo et al. 2015) to the typical
features of selfhood in dreams (Revonsuo 2005, 2006, Metzinger 2003,
2009) to the minimal conditions for experiencing oneself as a self in
dreams and what this tells us about minimal phenomenal selfhood in
general (Windt 2015a, 2018). Yet these different versions of the
simulation view are largely complementary and together have forged
unity in a field that was previously hampered by lack of agreement
about the definition of dreaming. They also integrate the philosophy
of dreaming and scientific dream research.



As so often in debates about dreaming, there is disagreement about
basic phenomenological questions. Revonsuo (2005) describes
self-experience including bodily experience in dreams as identical to
waking, whereas Metzinger (2003, 2009; see also Windt & Metzinger
2007) argues that important layers of waking self-experience (such as
autobiographical memory, agency, a stable first-person perspective,
metacognitive insight, and self-knowledge) are missing in nonlucid
dreams. He argues this is due to the cognitive and mnemonic deficit
that characterizes nonlucid dreams (cf. Hobson et al. 2000). Windt
(2015a) analyzes the range of cognitive and bodily self-experience in
dreams, both of which she describes as variable. She argues that in a
majority of cases, dreams are weakly phenomenally embodied states in
which bodily experience is largely related to movement sensations but
a detailed and integrated body representation is lacking; instead,
bodily experience in dreams is largely indeterminate (for an attempt
to test this empirically, see Koppehele-Gossel et al. 2016). She
proposes this is because dreams are also weakly functionally embodied
states, in which the specific pattern of bodily experience reflects
altered processing of bodily sensations (as in the illusion view). She
also analyzes instances of bodiless dreams, in which dreamers say they
experienced themselves as disembodied entities, to argue that
self-experience can be reduced to pure spatiotemporal-self-location
(Windt 2010); she proposes these cases can help identify the
conditions for the emergence of minimal phenomenal selfhood (Blanke
& Metzinger 2009; see also Metzinger 2013b).


## 5. Immorality and moral responsibility in dreams



How the phenomenology of dreaming compares to waking and what to say
about how the dream self relates to the waking self bears on questions
about the moral status of dreams. For Augustine (*Confessions*)
dreams were a cause of moral concern because of their
indistinguishability from waking life. What particularly worried him
about dreams of sexual acts was their vividness, as well as the
feeling of pleasure and seeming acquiescence or consent on the part of
the dreamer. He concluded, however, that the transition from sleep to
wakefulness involves a radical chasm, enabling the dreamer to awaken
with a clear conscience and absolving them from taking responsibility
for their dream actions.



What exactly Augustine thought the chasm between dreaming and waking
consists in allows for different interpretations (Matthews 1981).
Firstly, if the dream and waking self are not identical, then waking
Augustine is not morally responsible for dream-Augustine's
actions. Secondly, actions performed in dreams might be morally
irrelevant because they did not really happen. And thirdly, assuming
that moral responsibility requires the ability to act otherwise,
dreams provide no grounds for moral concern because we cannot refrain
from having certain types of dreams.



The issue of dream immorality may also present a choice point between
different accounts of moral evaluation. Where internalists assume the
moral status of a person's actions is entirely determined by internal
factors such as intentions and motives, externalists look beyond these
to the effects of actions. Driver (2007) argues that the absurdity of
dream immorality itself should count against purely internalist
accounts; yet she also acknowledges this absurdity is not a necessary
feature of dreams.



Central to the question of dream immorality is the status of dreams as
actions rather than mere behaviors. Mullane (1965) argues that while
we don't have full control over our dreams, they are not
completely involuntary either; as is the case for blushing,
considerable effort is required to attain control over our dreams and
in some cases they can even be considered as actions. That lucid dream
control is, to some extent, a learnable skill (Stumbrys et al. 2014)
lends some support to this claim.


## 6. The meaning of dreams and the functions of dreaming


### 6.1 The meaning of dreams



Philosophical discussions of dreaming tend to focus on (a) dream
deception and (b) questions about the ontology of dreaming, its moral
status, etc., that tend to intersect with dream skepticism. By
contrast, the main source of interest in dreams outside of philosophy
traditionally has been dream interpretation and whether dreams are a
source of knowledge and insight. Historically, the epistemic status of
dreams and the use of prophetic and diagnostic dreams was not just a
theoretical, but a practical problem (Barbera 2008). Different types
of dreams were distinguished by their putative epistemic
value. Artemidorus, for instance, used the term
*enhypnion* to refer to dreams that merely reflect the
sleeper's current bodily or psychological state and hence do not
merit further interpretation, whereas he reserved the term
*oneiron* for meaningful and symbolic dreams of divine
origin.



The practice of dream interpretation was famously attacked by
Aristotle in *On Prophecy in Sleep*. He denied that dreams are
of divine origin, but allowed that occasionally, small affections of
the sensory organs as might stem from distant events that cannot be
perceived in waking are perceptible in the quiet of sleep. He also
believed such dreams were mostly likely to occur in dullards whose
minds resemble an empty desert - an assessment that was not apt to
encourage interest in dreams (Kroker 2007: 37). A similarly negative
view was held by early modern philosophers who believed dreams were
often the source of superstitious beliefs (Hobbes 1651; Kant 1766;
Schopenhauer 1847).



In Freudian dream theory, dream interpretation once more assumed a
prominent role as the royal road to knowledge of the unconscious. This
was associated with claims about the psychic sources of dreaming.
Freud (1899) also rejected the influence of external or bodily
sources, as championed by contemporary proponents of somatic-stimulus
theory. 



In the neuroscience of dreaming, Hobson famously argued that dreams
are the product of the random, brain-stem driven activation of the
brain during sleep (Hobson 1988) and at best enable personal insights
in the same way as a Rorschach test (Hobson et al. 2000). Dennett
(1991) illustrates the lack of design underlying the production of
dream narratives through the "party game of
psychoanalysis", which involves an aimless game of
question-and-answer. In the game, players follow simple rules to
jointly produce narratives that can seem symbolic and meaningful, even
though no intelligent and deliberate process of narration was
involved.



Even if we grant that dreams are not messages from a hidden entity in
need of decoding, this does not imply that dream interpretation cannot
be a personally meaningful source of insight and creativity (Hobson
& Wohl 2005). Whether and under which conditions, and following
which methods, dream interpretation can lead to personally significant
insights is an empirical question that is only beginning to be
investigated systematically (see Edwards et al. 2013).



Finally, throughout history, views on the epistemic status of dreams
and the type of knowledge to be gained from dream interpretation
(e.g., knowledge about the future, diagnosis of physical illness, or
insights about one's current concerns) often changed in tandem
with views on the origin and sources of dreaming, which gradually
moved from divine origins and external sources, via the body, to the
unconscious, and finally to the brain.


### 6.2 The functions of dreaming



Different theories on the functions of dreaming have been proposed and
the debate is ongoing. An important distinction is between the
functions of sleep stages and the functions of dreaming.
Well-documented functions of REM sleep include thermoregulation and
the development of cortical structures in birds and mammals, as well
as neurotransmitter repletion, the reconstruction and maintenance of
little-used brain circuits, the structural development of the brain in
early developmental phases, as well as the preparation of a repertoire
of reflexive or instinctive behaviors (Hobson 2009). Yet none of these
functions are obviously linked to dreaming. An exception is
protoconsciousness theory, in which REM sleep plays an important role
in foetal development by providing a virtual world model
 even before the emergence of full-blown consciousness (Hobson 2009: 808).



Numerous studies have investigated the contribution of sleep to memory
consolidation, with different sleep stages promoting different types
of memories (Diekelmann et al. 2009; Walker 2009). However, only a few
studies have investigated the relationship between dream content and
memory consolidation in sleep (for a review, see Nielsen &
Stenstrom 2005). Dreams rarely involve episodic replay of waking
memories (Fosse et al. 2003). The incorporation of memory sources
seems to follow a specific temporal pattern in which recent memories
are integrated with older but semantically related memories (Blagrove
et al. 2011). Nielsen (2017) presents a model of how external and
bodily stimuli on one hand and short- and long-term memories on the
other hand form seemingly novel, complex, and dreamlike images at
sleep onset; he proposes these microdreams shed light on the formation
and sources of more complex dreams. There is also some evidence that
dream imagery might be associated with memory consolidation and task
performance after sleep, though this is preliminary (Wamsley &
Stickgold 2009, 2010; Wamsley et al. 2010).



Prominent theories on the function of dreaming focus on bad dreams and
nightmares. It has long been thought that dreaming contributes to
emotional processing and that this is particularly obvious in the
dreams of nightmare sufferers or in dreams following traumatic
experiences (e.g., Hartmann 1998; Nielsen & Lara-Carrasco 2007;
Levin & Nielsen 2009; Cartwright 2010; Perogamvros et al. 2013).
Based on the high prevalence of negative emotions and threatening
dream content, threat simulation theory suggests that the evolutionary
function of dreaming lies in the simulation of ancestral threats and
the rehearsal of threatening events and avoidance skills in dreams has
an adaptive value by enhancing the individual's chances of
survival (see Revonsuo 2000; Valli 2008). A more recent proposal is
social simulation theory, in which social imagery in dreams supports
social cognition, bonds, and social skills. (Revonsuo et al.
2015).



An evolutionary perspective can also be fruitfully applied to specific
aspects of dream phenomenology. According to the *vigilance
hypothesis*, natural selection disfavored the occurrence of those
types of sensations during sleep that would compromise vigilance
(Symons 1993). Dream sounds, but also smells or pains might distract
attention from the potentially dangerous surroundings of the sleeping
subject, and the vigilance hypothesis predicts that they only rarely
occur in dreams without causing awakening. By contrast, because most
mammals sleep with their eyes closed and in an immobile position,
vivid visual and movement hallucinations during sleep would not
comprise vigilance and thus can occur in dreams without endangering
the sleeping subject. Focusing on the stuff dreams are *not*
made of might then be at least as important for understanding the
function of dreaming as developing a positive account.



Finally, even if dreaming in general and specific types of dream
content in particular were found to be strongly associated with
specific cognitive functions, it would still be possible that dreams
are mere epiphenomena of brain activity during sleep (Flanagan 1995,
2000). It is also possible that the function of dreams is not knowable
(Springett 2019).



A particular problem for any theory on the function of dreaming is to
explain why a majority of dreams are forgotten and how dreams can
fulfill their putative function independently of recall. Crick and
Mitchinson (1983) famously proposed that REM sleep
"erases" or deletes surplus information and unnecessary
memories, which would suggest that enhanced dream recall is
counterproductive. Another problem is that dreaming can be lost
selectively and independently of other cognitive deficits (Solms 1997,
2000).



Some of the problems that arise for theories on the functions of
dreaming can be avoided if we do not assume that dreaming has a
specific function, separate from the function(s) of conscious wakeful
states. This depends on the broader taxonomy of dreaming in relation
to wakeful states. For example, if dreaming is continuous with waking
mind wandering, imagination, and/or own-body perception, we should not
expect it to have a unique function, but rather to express a similar
function as these wakeful states, perhaps to varying degrees. Nor
should we expect dreams to have a single function; the functions of
dreaming might be as varied and complex as those of consciousness, and
given the complexity of the target phenomenon, the failure to pin down
a single function should not be surprising (Windt 2015a).


## 7. Conclusions



Questions about dreaming in different areas of philosophy such as
epistemology, ontology, philosophy of mind and cognitive science, and
ethics are closely intertwined. Scientific evidence from sleep and
dream research can meaningfully inform the philosophical discussion
and has often done so in the past. The discussion of dreaming has also
often functioned as a lens on broader questions about knowledge,
morality, consciousness, and self. Long a marginalized area, the
philosophy of dreaming and of sleep is central to important
philosophical questions and increasingly plays an important role in
interdisciplinary consciousness research, for example in the search
for the neural correlates of conscious states, in conscious state
taxonomies, and in research on the minimal conditions for phenomenal
selfhood and conscious experience.