Delusion (Stanford Encyclopedia of Philosophy)


In recent years, delusions have attracted the attention of
philosophers in at least three distinct areas. Here is a summary of
the general issues that have been addressed and some examples of
specific debates for each of these areas.

1.1 Delusions in the philosophy of mind and the philosophy of psychology

In the philosophy of mind and the philosophy of psychology, there
have been various attempts to understand the cognitive processes
responsible for the formation of delusions, based on the assumption,
widely shared in cognitive neuropsychology, that understanding such
processes can lead to the formulation of more empirically sound
theories of normal cognition (see Marshall and Halligan 1996,
pp. 5–6; Langdon and Coltheart 2000, pp. 185–6). For
instance, let’s assume that delusions are pathological beliefs. How do
they come about? Do people form delusional beliefs as a response to
bizarre experiences? Do they form delusional beliefs because they
have some reasoning deficit?

As the above questions already suggest, the study of delusions raises
conceptual questions about intentionality, and about the relationship
between intentionality, rationality and self-knowledge. Moreover, it
invites us to reconsider the interaction between perception,
cognition, and intentional behavior. One basic question is what comes
first, the experience or the belief (see Campbell 2001): are delusions
bizarre convictions that alter one’s way of seeing the world, or are
they hypotheses formulated to account for some unusual experiences,
and then endorsed as beliefs? Another debated issue is whether
delusions should be characterized as beliefs at all, given that they
share features with acts of imagination (Currie 2000), desires (Egan
2009) and perceptions (Hohwy and Rajan 2012). Can delusions be beliefs
if they present significant deviations from norms of rationality, and
are often neither consistent with a person’s beliefs nor responsive to
the available evidence? Bayne and Pacherie (2005) and Bortolotti
(2009) offer defenses of the doxastic nature of delusions, but this is
still a hotly debated issue. An interesting position defended
by Schwitzgebel (2012) is that delusions are in-between states
(neither beliefs nor non-beliefs), because they match only in part the
dispositional profile of beliefs. Schwitzgebel’s position has been
challenged by philosophers who argue that delusions play a belief-role
in explaining and predicting intentional action (see Bortolotti 2012;
Bayne and Hattiangadi 2013).

Another strand of investigation developing in this area concerns
the possible failures of self knowledge exhibited by people with
delusions. There are several manifestations of poor knowledge of the
self in delusions (see Kircher and David 2003; Amador and David
1998). People reporting delusions of passivity may not recognize a
movement or a thought as their own, and thus have a distorted sense of
their personal boundaries (e.g., Stephens and Graham 2000). People
with delusions may act or feel in a way that is incompatible with the
content of their delusions, or be unable to endorse the content of
their delusion with reasons that are regarded by others as good
reasons (e.g., Gallagher 2009; Bortolotti and Broome 2008, 2009;
Fernández 2010). Finally, people reporting delusions may
encounter difficulties in remembering their experienced past and in
projecting themselves into the future, because they construct
unreliable self narratives (e.g., Gerrans 2009, 2014).

1.2 Delusions in the philosophy of psychiatry

In addition to the literature on the etiology of delusions and their
status as beliefs, there is also a growing literature in the
philosophy of psychiatry on other aspects of the nature of delusions
and on the impact of delusions on people’s mental health. This
literature aims at addressing the conceptualization of delusional
experience and of delusional beliefs in the wider context of
psychiatric research and clinical practice. General debates in the
philosophy of psychiatry are often applied to delusions more
specifically, such as whether delusions are natural kinds (e.g.,
Samuels 2009), and whether they are a pathological phenomenon (e.g.,
Fulford 2004).

If we acknowledge that delusions are pathological, there are at least
six possible non-exclusive answers to what makes delusions

  1. Delusions are pathological because they present themselves as
    what they are not. They resemble beliefs but do not share some of the
    core features of beliefs such as action guidance, and are irrational
    to a higher degree than or in a qualitatively different way from
    irrational beliefs (for a discussion of aspects of this view, see
    Currie and Jureidini 2001 and Frankish 2009).

  2. Delusions are pathological because they are signs that the
    person inhabits a fictional, non-actual reality and no longer shares
    some fundamental beliefs and practices with the people around her (for
    different versions of this view, see Stephens and Graham 2004 and
    2006; Sass 1994; Gallagher 2009; Rhodes and Gipps 2008).

  3. Delusions are pathological because they are puzzling and
    unsettling – and defy folk-psychological
    expectations –. This also makes them difficult to
    rationalize and interpret (this idea is explored in Campbell
    2001 and Murphy 2012).

  4. Delusions are pathological because (differently from many
    irrational beliefs) they negatively affect a person’s well-being
    causing impaired social functioning, social isolation and withdrawal
    (see Garety and Freeman 1999 for a multidimensional account of
    delusions, and Bolton 2008 for a harm-related account of mental
    illness in general).

  5. Delusions are pathological because they have forensic
    implications, that is, implications for judgements about whether
    agents can be held legally accountable for their actions. Hohwy and
    Rajan (2012) argue that we tend to attribute delusions when we notice
    significant impairments in decision-making, autonomy, and

  6. Delusions are pathological because of their
    etiology. Differently from other beliefs, they are produced by
    mechanisms that are dysfunctional or defective. For instance, the
    process of their formation may be characterized by perceptual
    aberrations, reasoning biases or deficits.

The challenge for (i) is to account for the difference
in kind between the irrationality of common beliefs that are
ungrounded and resistant to change (such as superstitious beliefs or
beliefs in alien abductions) and the irrationality of delusions. There
is abundance of evidence that delusional phenomena are widespread in
the normal population, which suggests that a sharp dichotomy between
the normal and the pathological would be at best a simplification (see data in
Maher 1974, Johns and van Os 2001, and Bentall 2003).

Accounts in (ii) and (iii) may be plausible for some delusions that
appear to defy commonsense and are accompanied by a certain type of
heightened experience, but do not seem to apply equally well to more
mundane delusions such as jealousy or persecution. Moreover, it is not
always obvious that ascribing a delusion as a belief to someone makes
the behavior of that person particularly difficult to explain or to

The view described in (iv) is very attractive because it captures the
distinction between delusions and irrational beliefs in terms of their
effects on other aspects of a person’s psychological and social
life. However, using the notions of well-being and harm in accounts of
delusions can be problematic, since it is possible for some people to
live with the delusion in a way that is preferable to living without
the delusion: ceasing to believe that one is a famous TV broadcaster
after many years, and starting to accept that one has been mentally
unwell instead, can cause low self esteem leading to depression and
suicidal thoughts. Indeed, in the philosophical and psychiatric
literature there have been recent explorations of the idea that some
delusions may be adaptive in some sense, psychologically,
biologically, and even epistemically (McKay and Dennett 2009, Fineberg
and Corlett 2016, Bortolotti 2016).

Challenges for a forensic account of delusions in (v) lie in the
heterogeneity of the behavior exhibited by those who experience
delusions. Although some delusions can be accompanied by severe
failures of autonomous decision-making and give rise to action for
which the agent is not held accountable, it is not obvious that these
are generalisable phenomena. Does the mere presence of delusions
indicate lack of autonomy or responsibility? Broome et
. (2010) and Bortolotti et al. (2014) discuss case
studies raising interesting issues about the role of delusions in
criminal action.

The etiological answer to the question why delusions are pathological
in (vi) needs to be better explored. So far, the consensus seems to be
that reasoning biases affect normal reasoning, and are not present
only in people with delusions. Perceptual aberrations can explain the
formation of some delusions, but are not always a core factor in the
formation of all delusions. A problem with the hypothesis evaluation
system involved in the formation of beliefs may be at the origin of
all delusions, but there is no agreement as to whether the problem is
a permanent deficit or a performance error. Thus, it is not clear
whether etiological considerations can support a categorical
distinction between pathological and non-pathological beliefs. The
theory that delusions are due to a disruption of prediction-error
signals may be able to vindicate this approach, although it is not
clear what the link would be between such a disruption and the
pathological nature of the beliefs adopted as a result of it.

1.3 Moral psychology and neuroethics

Moral psychology and neuroethics investigate the implications of the
debates on the nature of delusions in the philosophy of mind and the
philosophy of psychiatry for the type of participation in the moral
community to which people with delusions are entitled. This includes
the attempt to understand better how people’s rights and
responsibilities are affected by their having delusions. For instance,
it is important to determine when people with delusions no longer have
the capacity to consent to being treated in a certain way, and to
safeguard their interests by ensuring that they receive good care. It
is also important to understand whether they can be regarded as morally
responsible for their actions if they commit acts of violence or other
crimes that can be motivated by their believing the content of their

As a consequence of the failures in rationality and self knowledge
that may characterize people with delusions in the acute phase of their
mental illness, they may appear as if they were ‘in two
minds’, and they may not always present themselves as unified
agents with a coherent set of beliefs and preferences (e.g., Kennett and
Matthews 2009). As a result, they may be (locally or temporally) unable
to exercise their capacity for autonomous thought and action.

We saw some examples of delusions, but no definition yet. How are
delusions defined and classified?

2.1 Defining delusion

Commonly used definitions of delusions make explicit reference to
their surface features rather than to the underlying mechanisms
responsible for their formation. Surface features refer to the
behavioral manifestations of the delusions, and are often described in
epistemic terms, that is, their description involves the concept of
belief, truth, rationality or justification (e.g., delusions are
beliefs held with conviction in spite of having little empirical
support). According to the Glossary in the Diagnostic and
Statistical Manual of Mental Disorders
(DSM-IV 2000, p. 765 and
DSM-5 2013, p. 819), delusions are false beliefs based on incorrect
inference about external reality that persist despite evidence to the

Delusion. A false belief based on incorrect inference about external
reality that is firmly sustained despite what almost everyone else
believes and despite what constitutes incontrovertible and obvious
proof or evidence to the contrary. The belief is not one ordinarily
accepted by other members of the person’s culture or subculture
(e.g., it is not an article of religious faith). When a false belief
involves a value judgment, it is regarded as a delusion only when the
judgment is so extreme as to defy credibility.

Philosophers interested in the nature of delusions have asked a number
of questions which highlight the weaknesses of the DSM definition. For
instance, how can we tell delusions apart from other pathologies
involving cognitive impairments or deficits? How can we distinguish
delusions from non-pathological, but similarly false or unjustified
beliefs? These questions aim at capturing both what is distinctive
about delusions, and what makes them pathological.

Delusions are generally accepted to be beliefs which (a) are held
with great conviction; (b) defy rational counter-argument; (c) and
would be dismissed as false or bizarre by members of the same
socio-cultural group. A more precise definition is probably impossible
since delusions are contextually dependent, multiply determined and
multidimensional. Examplars of the delusion category that fulfil all
the usual definitional attributes are easy to find, so it would be
premature to abandon the construct entirely. Equally, in everyday
practice there are patients we regard as deluded whose beliefs in
isolation may not meet standard delusional criteria. In this way a
delusion is more like a syndrome than a symptom. (Gilleen and David
2005, pp. 5–6)

Counterexamples are easily found to the DSM definition of delusion:
there are delusions that do not satisfy all of the proposed criteria,
and there are irrational beliefs that do, even though they are not
commonly regarded as delusional. Coltheart summarizes the main problems
with the DSM definition:

1. Couldn’t a true belief be a delusion, as long as the
believer had no good reason for holding the belief? 2. Do delusions
really have to be beliefs—might they not instead be imaginings that
are mistaken for beliefs by the imaginer? 3. Must all delusions be
based on inference? 4. Aren’t there delusions that are not about
external reality? ‘I have no bodily organs’ or ‘my
thoughts are not mine but are inserted into my mind by others’
are beliefs expressed by some people with schizophrenia, yet are not
about external reality; aren’t these nevertheless still
delusional beliefs? 5. Couldn’t a belief held by all members of
one’s community still be delusional? (Coltheart 2007, p.

The Diagnostic and Statistical Manual of Mental Disorders has
been updated recently and although no changes appear in the Glossary,
some interesting shifts can be noted in the description of delusions
which appear in the section on schizophrenia (compare DSM-IV, p. 275
and DSM-IV-TR p. 299 with DSM-5, p. 87). The new description seems to
take into account some of the issues identified by Coltheart and
others. For instance, in the DSM-5 delusions are described not as
false, but as “fixed beliefs that are not amenable to change in
light of conflicting evidence”. Leaving the details aside, some
general comments apply to the style of the DSM definitions and
descriptions of delusions. In so far as delusions are defined and
described as irrational beliefs, it is difficult for them to be
uniquely identified because their epistemic ‘faults’ are
shared with other symptoms of psychiatric disorders, and with
non-pathological beliefs. But definitions such as the ones in the DSM
cannot probably be expected to provide necessary and sufficient
conditions for the phenomena they aim to define. At best, they can
prove diagnostically useful and guide further research by conveniently
delimitating an area of investigation worth pursuing.

A widespread critique of the DSM definition is that not enough weight
is given to the consequences of having the delusion for the well-being
of the person reporting it. Some recent definitions of delusion make
more explicit reference to “disrupted functioning”(e.g.,
McKay et al. 2005a, p. 315). Freeman (2008, pp. 24–26)
highlights the multi-dimensional nature of delusions and lists among
the main characteristics of delusions not just that delusions are
unfounded, firmly held, and resistant to change, but also that they
are preoccupying and distressing, and that they interfere with the
social dimension of a person’s life.

2.2 Types of delusion

2.2.1 Functional versus organic

Delusions used to be divided into functional and
organic. Now the distinction is regarded by most obsolete, at
least in its original characterization. A delusion was called
‘organic’ if it was the result of brain damage (usually due
to injuries affecting the right cerebral hemisphere). A delusion was
called ‘functional’ if it had no known organic cause and
was explained primarily via psychodynamic or motivational factors. It
has become more and more obvious with the development of
neuropsychiatry that the two categories overlap. Today, the received
view is that there is a biological basis for all types of delusions,
but that in some cases it has not been identified with precision yet.
Some studies have reported very little difference between the
phenomenology and symptomatology of delusions that were once divided
into organic and functional (Johnstone et al. 1988).

2.2.2 Monothematic versus polythematic

As we saw, in persecutory delusions, people believe that they are
followed and treated with hostility, and that others want to harm them.
In delusions of mirrored-self misidentification, people usually
preserve the capacity to recognize images in the mirror as reflections,
but do not recognize their own face reflected in the mirror and come to
think that there is a person in the mirror, a stranger who looks very
much like they do. In either case, the delusion is resistant to
counterevidence and has pervasive effects on one’s life. One of
the differences is that persecutory delusions are
polythematic, that is, they extend to more than one theme,
where the themes can be interrelated. Delusions of mirrored-self
misidentification are monothematic, and apart from the content
of delusion itself, no other (unrelated) bizarre belief needs to be
reported by the same person. Thus, a person who systematically fails to
recognize her image in the mirror and comes to think that there is a
person identical to her following her around (as in mirrored-self
misidentification), but has no other unusual beliefs, has a
monothematic delusion. Other examples of monothematic delusions often
referred to in the philosophical literature are Capgras and Cotard. The
Capgras delusion involves the belief that a dear one (a close relative
or the spouse) has been replaced by an impostor. The Cotard delusion
involves the belief that one is disembodied or dead. Delusions of
persecution are very common polythematic delusions. A person who
believes that she is surrounded by alien forces and that they control her own actions
and are slowly taking over people’s bodies might have a number of
different delusions (persecution and alien control). These delusions
are interrelated and are manifest in the interpretation of most events
occurring in the person’s life. Other examples of delusions that
affect many aspects of one’s cognitive life are the belief that
one is a genius but is often misunderstood by others (grandeur), and
the belief that one is loved by a famous or powerful person

2.2.3 Circumscribed versus elaborated

Monothematic delusions tend to be circumscribed whereas
polythematic delusions tend to be elaborated (see Davies and
Coltheart 2000 for more detailed explanation and examples). The
distinction between circumscribed and elaborated delusions is relevant
to the level of integration between delusions and a person’s
other intentional states and to the extent to which the person’s
endorsement of the delusion is manifested in verbal reports and
observable behavior. Delusions might be more or less circumscribed. A
delusion is circumscribed if it does not lead to the formation of other
intentional states whose content is significantly related to the
content of the delusion, nor does it have pervasive effects on the
behavior of the person reporting the delusion. For instance, a person
with Capgras who believes that his wife has been substituted by an
impostor but shows no preoccupation for his wife and does not go and
look for her, appears to have a circumscribed delusion. A delusion can
be elaborated, if the person reporting the delusion draws consequences
from the delusional state and forms other beliefs that revolve around
the theme of the delusion. For instance, a person with Capgras can
develop paranoid thoughts related to the content of the delusion, along
the lines that the impostor has evil intentions and will cause harm
when the occasion presents itself.

2.2.4 Primary versus secondary

Depending on whether the delusion seems to be reported on the basis
of some reasons, and defended with arguments, delusions can be
described as primary or secondary. The traditional
way of distinguishing primary from secondary delusions relied on the
notion that primary delusions ‘arise out of nowhere’
(Jaspers 1963). This traditional characterization of the distinction
has been found problematic, because it is difficult to establish
whether there are antecedents of the delusion in a person’s line
of reasoning, and for other methodological and clinical reasons (e.g.,
Miller and Karoni 1996, p. 489). New readings of the distinction have
been provided in the recent philosophical literature on delusions,
where the need arises for distinguishing between people who can endorse
the content of their delusions with reasons, and people who cannot
(e.g. Bortolotti and Broome 2008 talk about authored and un-authored
delusions; and Aimola Davies and Davies 2009 distinguish between
pathologies of belief and pathological beliefs on similar lines).

There are several theoretical approaches to delusion formation which
attempt to explain the surface features of delusions by reference to
abnormal experiences, reasoning biases, neuropsychological deficits,
motivational factors, and prediction error, but the task of describing
the behavioral manifestations of delusions, and reconstructing their
etiology is made difficult by the variation observed both in the form
and in the content of delusions.

When the distinction between functional and organic delusion was
still widely accepted, functional delusions were primarily explained on
the basis of psychodynamic factors, whereas organic delusions primarily
received a neurobiological explanation. At the present stage of
empirical investigation in the formation of delusional states, the
received view is that all delusions are due to neuropsychological
deficits, which might include motivational factors.

3.1 Neuropsychological and psychodynamic accounts of delusion

According to psychodynamic accounts, there needs to be no
neurobiological deficits and delusions are caused by motivational
factors alone. For instance, delusions of persecution would be
developed in order to protect one from low self-esteem and depression,
and would be due to the attribution of negative events to some
malevolent other rather than to oneself. The delusion would be part of
a defense mechanism. Other delusions, such as Capgras, have
also received a psychodynamic interpretation: a young woman believes
that her father has been replaced by a stranger looking just like him
in order to make her sexual desire for him less socially
objectionable. In this way, the delusion would have the function to
reduce anxiety and sense of guilt. Psychodynamic accounts of the
Capgras delusion have been strongly criticized on the basis of recent
findings about the type of brain damage that characterizes people with
Capgras and affects their face recognition system. Psychodynamic
accounts of other delusions that are supposed to play a defensive or
self-enhancing role (e.g., persecution, anosognosia and erotomania)
are still very popular.

Neuropsychological accounts of delusions offer very satisfactory
accounts of some delusions, as one can often identify with some
precision the damaged region of the brain and the causal link between
the damage and the formation of the delusion. Neuropsychological
accounts of other delusions—once regarded as
‘functional’—are also being developed and
explored. For some delusions, hybrid accounts have been proposed,
where a combination of different factors (including motivation)
significantly contribute to the formation of the delusion (e.g. McKay
et al. 2007). One such case seems to be the Reverse Othello
Syndrome, the delusion that a spouse or romantic partner is still
faithful when this is no longer the case. The belief can be regarded
as a defense against the suffering that the acknowledgement of the
infidelity of one’s partner would cause (see example in Butler 2000 as
cited and discussed by McKay et al. 2005a, p. 313).

According to popular neuropsychological accounts, delusions are the
result of a cognitive failure, which can be an abnormal perceptual
experience (Maher 1974); an abnormal experience accompanied by milder
dysfunctions such as reasoning biases (Garety and Freeman 1999; Garety
et al. 2001); a breakdown of certain aspects of perception
and cognition including a deficit in hypothesis evaluation (Langdon
and Coltheart 2000); or a failure of predictive coding (Fletcher and
Frith 2009; Corlett et al. 2010).

In the two-factor theory framework, an abnormal event is
responsible for the formation of the delusion. The young woman who
thinks that her father has been replaced by an impostor would form
this belief because she has reduced autonomic response to familiar
faces, and this affects her capacity to recognize the face of the man
in front of her as her father’s face, even if she can judge that the
face is identical (or virtually identical) to that of her father. But
this abnormal event (reduction of autonomic response) is not the only
factor responsible for the formation of the delusion. In order to
explain why the thought that a dear one has been replaced by an
impostor is adopted as a plausible explanation of the abnormal event,
one also needs to advocate a deficit at the level of hypothesis
evaluation (Coltheart 2007), or the presence of exaggerated
attributional or data-gathering biases, such as the tendency to
‘jump to conclusion’ on the basis of limited evidence
(Garety and Freeman 1999).

According to the prediction-error theory, expectations are formed
about experience, and greater attention is paid to those events which
defy expectations. The discrepancy between what is expected and the
information taken in is an important part of the way learning
occurs. When expectations are not met, a prediction error is coded,
and the representation of the world is updated accordingly. A
prediction-error signal is disrupted when events invested of special
significance (in psychosis this may be due to dopamine dysregulation)
cause one to update one’s current (correct) beliefs about reality. The
woman who see her father and does not get the usual autonomic response
experiences an unexpected event which gives rise to a prediction
error. The reaction to the signal is to attempt an explanation of the
unexpected event (’That man is not my father!’). This results in the
formation of a delusion (Corlett et al. 2007).

3.2 Bottom-up versus top-down theories of delusion

Another distinction, introduced and developed in the philosophical
literature on delusions, is between bottom-up and
top-down theories, where these labels are meant to refer to
the direction of the causal relation between experience and belief in
the formation of the delusion. Bottom-up theorists argue that the
direction of causal explanation is from the experience to the
belief. Top-down theorists argue that the direction of causal
explanation is from the belief to the experience. Notice that not
everybody finds the distinction useful. For instance, Hohwy and
Rosenberg (2005) and Hohwy (2004) argue that the distinction loses its
appeal in the framework proposed by prediction-error theorists given
that delusion formation involves both bottom-up and top-down
processes. A person’s prior expectations affect the way in which the
perceptual signals are processed and give rise to unusual
experiences. Then, the unusual experiences go through reality testing
and are subject to further interpretation, after which they become a
central factor in the formation of the delusional belief.

For bottom-up theorists, delusions involve modifications of the belief
system that are caused by ‘strange experiences’ due to
organic malfunction (Bayne and Pacherie 2004a; Davies et
. 2001). For instance, I experience people watching me with
suspicion or hostility, and as a result I form the hypothesis that
they want to harm me; or something does not feel right when I see my
sister’s face, and as a result I come to believe that the person I am
looking at is not really my sister but an impostor.

The proximal cause of the delusional belief is a certain highly
unusual experience (Bayne and Pacherie 2004a, p. 2).

What would top-down theorists say about the same examples? I believe
that people want to harm me, and as a result I perceive them as
looking at me malevolently; or I believe that someone looking almost
identical to my sister has replaced her, and as a result the person
claiming to be my sister doesn’t look to me as my sister does. The
top-down thesis about delusion formation has been proposed especially
for monothematic delusions such as Capgras (Campbell 2001; Eilan 2000)
and for delusions of passivity, when people report that there are
external influences on their thoughts and actions (Sass 1994; Graham
and Stephens 1994; Stephens and Graham 2000).

[D]elusion is a matter of top-down disturbance in some fundamental
beliefs of the subject, which may consequently affect experiences and
actions (Campbell 2001, p. 89).

Both bottom-up and top-down
theories face challenges: whereas top-down theorists need to account for where the
belief comes from, and why it is so successful in affecting perceptual
experiences, bottom-up theorists are pressed to explain why people tend
to endorse a bizarre hypothesis to explain their unusual experiences,
given that hypotheses with higher probability should be available to

Within the bottom-up camp, further divisions apply. For some, it is
correct to say that the delusional belief explains the
experience. Others claim that the delusion is an endorsement
of the experience. According to the explanationist account (Maher 1999;
Stone and Young 1997), the content of experience is vaguer than the
content of the delusion, and the delusion plays the role of one
potential explanation for the experience. For instance, in the Capgras
delusion, the experience would be that of someone looking very much
like my sister but not being my sister. The delusion would be an
explanation of the fact that the woman looks like my sister, but her
face feels strange to me: the woman must be an impostor. In persecution, the
experience would be that of some people as being hostile, and the
delusion would be an explanation why they seem hostile: they have an
intention to harm me. This account leaves it open that the same
experience could have been explained differently (i.e., without any
appeal to the delusional hypothesis).

According to the rival account, the endorsement account (Bayne and
Pacherie 2004a; Pacherie et al. 2006), the content of the
experience is already as conceptually rich as the content of the
delusion. The delusion is not an explanation of the experience, but an
endorsement of it: the content of the experience is taken as veridical
and believed. In Capgras, the experience is that of a woman looking
very much like my sister but being an impostor, and when the experience
is endorsed, it becomes the delusional belief that my sister has been
replaced by an impostor. In persecution, the experience is that of
people having an intention to harm me, and when it is endorsed, it
becomes the delusional belief that those people want to harm me.

Both versions of the bottom-up theory seem to imply that the
delusion starts with a conscious experience, or better, with an
experience whose content is available to a person as something to be
explained or to be endorsed. But Coltheart (2005b) suggests instead
that in the typical case the process of delusion formation starts with
an event that a person is not aware of, such as the absence of an
autonomic response.

3.3 One-factor, two-factor and prediction-error theories of delusion formation

If the delusional belief comes from the experience, why is the
delusional hypothesis preferred to more probable and plausible
hypotheses (in the explanationist language), or why is the content of the
experience endorsed in spite of low probability and plausibility (in
the language of the endorsement account)? There are several replies
to this objection in the literature, which have given rise to
competing theories of delusion formation. Bottom-up theorists can be
divided in those who think that the unusual experience is sufficient
for the formation of the delusion (one-factor theorists), and
those who think that the unusual experience is only one factor in the
formation of the delusion (two-factor theorists).

For some one-factor theorists (Maher 1974), the delusion is a
reasonable hypothesis given the strangeness of the
experience, or the strange experience is in a sensory modality or at a
processing stage where further reality testing is not available (Hohwy
and Rosenberg 2005). But other one-factor theorists (e.g. Gerrans
2002a) argue that, although it may be reasonable to articulate a
delusional hypothesis, it is not rational to maintain it in the face
of counterevidence. For two-factor theorists (Davies et
. 2001; Stone and Young 1997), the delusion is formed in order
to explain a puzzling experience or a failed prediction, but the
presence of the experience or the failed prediction is not sufficient
for the formation of the delusion. The mechanism responsible for the
formulation of the delusional hypothesis must be affected by reasoning
biases or deficits. Recent developments of this theory have been
offered by Aimola Davies and Davies 2009, by Coltheart et
. 2010, and by Davies and Egan 2013.

Thus, there are three main positions as to whether reasoning is
impaired in people with delusions: (1) it is not impaired at all or
the apparent impairment is due to a performance error rather than to a
limitation of reasoning competence; (2) it is impaired due to a
hypothesis evaluation deficit, and possibly reasoning biases; (3) it
is impaired due to reasoning biases only. Although the predominance of
certain reasoning styles and the presence of reasoning biases in
people with delusions have been studied extensively, the available
evidence does not seem to clearly prioritise one of the three options
above. It is difficult at this stage of theoretical development to
establish whether a certain reasoning “mistake” is due to
a failure of competence or a failure of performance, or to specify
exactly what processes are involved in the hypothesis evaluation

By reference to monothematic delusions, Max Coltheart explains the
two main factors involved in the formation of delusions as follows:

  1. There is a first neuropsychological impairment that presents the
    patient with new (and false data), and the delusional belief formed is
    one which, if true, would explain these data. The nature of this
    impairment varies from patient to patient.
  2. There is a second neuropsychological impairment, of a belief
    evaluation system, which prevents the patient from rejecting the newly
    formed belief even though there is much evidence against it. This
    impairment is the same in all people with monothematic delusions.
    (Coltheart 2005b, p. 154)

In Davies et al. (2001) and Coltheart (2007), factor two is
described in more details. First there is the generation of a
hypothesis which serves as an explanation of the experience or an endorsement
of the content of the experience. Second, there is a failure in
rejecting the hypothesis, even when it is not supported by the
available evidence and it is implausible given the person’s
background beliefs—such a failure is probably due to frontal
right hemisphere damage. Finally the hypothesis is accepted, attended
to and reported, and can be subject to further (personal-level)
evaluation when counterevidence emerges. When it is endorsed, the
hypothesis is regarded as more plausible, more probable, and more
explanatory than relevant alternatives. This influential account of the
neuropsychology of delusions appeals to general mechanisms of belief
formation, namely hypothesis generation and evaluation, and is
compatible both with the view that people with delusions have
‘non-optimal hypothesis-testing strategies’ (Kihlstrom and
Hoyt 1988, p. 96) and with the thesis that these sub-optimal
strategies may be caused by damage to the right hemisphere
(Ramachandran and Blakeslee 1998) which would be responsible for
examining the fit between hypothesis and reality.

A similar story is told for polythematic delusions, self-deception,
and delusion- and confabulation-like episodes in the normal population,
although in such cases a single deficit could be at the origin of the
reported belief (see McKay et al. 2005a). Experiential
information is misinterpreted due to attentional or data-gathering
biases that affect the generation of hypotheses or to powerful
motivational factors.

The prediction-error theory of delusion formation differs from the
two-factor account in that it is not wedded to the doxastic nature of
delusions and it focuses on the similarities between delusions and
perceptual illusions (Hohwy 2012, 2013). There need be no specific
reasoning deficit which contributes to the formation of the delusion,
but a disruption in the coding of the prediction error, which causes
accurate beliefs to be revised, with the result that they become
inaccurate. The choice between 1-factor and 2-factor theories depends
on a sharp distinction between perception and reasoning. The
predictive error approach to delusions assumes an architecture on
which this distinction is harder to draw, since even the most
paradigmatically perceptual processing draws on predictions.

This section focuses on three debates that have animated the
philosophical literature on delusions in recent years. They can all be
seen as attempts to examine the extent to which the reasoning patterns
and styles exhibited by people with delusions are continuous with
those exhibited by people who have no known pathology of

4.1 Are delusions irrational?

There is no doubt that the definitions of delusions in DSM-IV and
DSM-5 characterise delusions as irrational beliefs. However, in the
philosophical literature on delusions, the status of delusions as
irrational beliefs does not go unchallenged. Are delusions really

In a number of influential papers Brendan Maher (1974, 1988, 1999,
2003) argues that delusions are not ill-formed beliefs, and that there
is nothing irrational in the relationship between the evidence
supporting the delusional hypothesis and the formation of such a
hypothesis. According to Maher, the abnormality of the delusion is
entirely due to the abnormality of the experiences on the basis of
which the delusion is formed. By reference to Maher’s model,
Blaney (1999) describes delusions as ‘false but
reasonable’. Some difficulties have been identified with this
strategy. A first difficulty is that there seem to be people who suffer
from the same type of brain damage, and plausibly have the same
experience, as the people who develop the delusion, but do not accept
any delusional hypotheses. How can these people avoid forming a
delusion? One possible answer is that those who have strange
experiences and do not form the delusion have hypothesis-evaluation
mechanisms that work efficiently, and thus end up rejecting hypotheses
with low probability and plausibility. But those who have strange
experiences and do form the delusion are instead affected by an
additional problem, a deficit at the level of hypothesis evaluation,
which can be conceived as a failure of rationality.

On Maher’s view, […] [i]t follows that anyone who has
suffered neuropsychological damage that reduces the affective response
to faces should exhibit the Capgras delusion; anyone with a right
hemisphere lesion that paralyzes the left limbs and leaves the subject
with a sense that the limbs are alien should deny ownership of the
limbs; anyone with a loss of the ability to interact fluently with
mirrors should exhibit mirrored-self misidentification, and so on.
However, these predictions from Maher’s theory are clearly
falsified by examples from the neuropsychological literature (Davies
et al. 2001, p. 144).

Another difficulty with Maher’s original account of delusions
as ‘false but reasonable’ is that, even if the abnormality
of the experience were to satisfactorily explain the acceptance of the
delusional hypothesis and the formation of the delusion, this would not
be sufficient to guarantee that the behavior of people with delusions
is overall rational. We would still have to explain why delusions are
maintained in the face of counterevidence once the delusional
hypothesis has been formed and endorsed (see Gerrans 2002a). One aspect
of the notion of rationality for beliefs is that people are disposed to
revise or abandon beliefs that seem to be in conflict with the acquired
evidence. The “incorrigibility” of delusions speaks in
favor of their being held irrationally.

Let’s concede that maintaining delusions (if not forming them)
is irrational. Which norms of rationality are violated by the obstinate
attachment to a delusional hypothesis? One norm that does seem to be
infringed by delusions is consistency, where this is intended both as
consistency between the delusion and the person’s other beliefs,
and consistency between the delusion and the person’s

Rationality is a normative constraint of consistency and coherence
on the formation of a set of beliefs and thus is prima facie violated
in two ways by the delusional subject. First she accepts a belief that
is incoherent with the rest of her beliefs, and secondly she refuses to
modify that belief in the face of fairly conclusive counterevidence and
a set of background beliefs that contradict the delusional belief
(Gerrans 2000, p. 114).

Delusions do not seem to respect the idea that the belief system
forms a coherent whole and that adjustments to one belief will require
adjustments to many others (Young 2000, p. 49).

In the course of the same interview, a woman may claim that her
husband died four years earlier and was cremated and that her husband
is a patient in the same hospital where she is (Breen et al.
2000, p. 91). In Capgras delusion, people may worry about the
disappearance of their loved one, but also be cooperative and even
flirtatious with the alleged impostor (see Lucchelli and Spinnler
2007). This suggests that delusions do not always give rise to
appropriate action (Bleuler 1950; Sass 2001), although they must be
reported either spontaneously or after questioning, or they could not
be diagnosed as delusions. How can we square people’s apparent strong
conviction in the content of the delusion with their failure to act on
it? One hypothesis is that the content of the delusion is not
genuinely believed. Another hypothesis is that the content of the
delusion is genuinely believed but not coverted into action, because
the person fails to acquire or maintain the motivation to act (this
would be consistent with negative symptoms of schizophrenia, see
Bortolotti and Broome 2012).

One should not be too impressed by ‘behavioural
inertia’ in people with delusions, as there are many examples of
people acting on their delusions. Affected by perceptual delusional
bicephaly, the delusion that one has two heads, a man who believed
that the second head belonged to his wife’s gynecologist attempted to
attack it with an axe. When the attack failed he attempted it to shoot
it down—as a consequence he was hospitalized with gunshot
wounds (Ames 1984). Cases of Cotard delusion have been reported where
people stop eating and bathing themselves as a consequence of
believing that they are dead (Young and Leafhead 1996).

Other possible violations of norms of rationality come from the
relationship between the content of the delusion and the available
evidence. Resistance to revising or abandoning the delusion in the face
of powerful counterevidence or counterargument is a sign of
irrationality in normal and abnormal cognition alike: people with
delusions ignore relevant evidence or attempt to defend their beliefs
from apparent objections with obvious confabulations. Often these
attempts are deeply perplexing, as the reasons offered for believing in
the content of their delusions do not seem to be good reasons: in one
of the examples of delusions offered at the start, a woman
incorrectly believed that a man was in love with her and claimed that
he was sending her secret love messages hidden in the license plates on
cars of a certain state.

Thus, delusions may be inconsistent with a person’s beliefs
and behavior, are typically unresponsive to both counterevidence and
counterargument, and are often defended by weak evidence or argument.
The empirical literature suggests
that the reasoning performance of people with delusions reflects
data-gathering and attribution biases. For instance, it has been argued
that people with delusions ‘jump to conclusions’; they need
less evidence to be convinced that a hypothesis is true (Garety 1991;
Huq et al. 1988; Garety and Freeman 1999), and are more hasty
in their decisions (Moritz and Woodward 2005; Fine et al.
2007). Other biases have also been noted: people with delusions of
persecution tend to attribute the responsibility of negative events to
other people (e.g., McKay et al. 2005b); in the Cotard
delusion there seems to be a tendency to attribute the responsibility
of negative events to oneself (Young and Leafhead 1996; Gerrans 2000;
McKay and Cipolotti 2007). There are further studies suggesting that
people with delusions are worse than controls at inhibiting the
evidence of their senses when it conflicts with other things they know
(Langdon et al. 2008b) and that they have an accentuated need
for closure which comprises a desire for clarity and structure (see
Kruglanski 1989, p. 14). These data are not by themselves sufficient to
support the view that delusions are irrational, but show interesting
deviations from statistically normal performance in the behavior of
people with delusions.

A very recent debate relevant to the rationality of delusions concerns
the step from abnormal data gathered via perception and the delusional
belief. (This is primarily an issue that emerged among two-factor
theorists, so the language used below is acceptable to them, but the
problem can be reformulated in terms that are friendly to
prediction-error theorists.) Coltheart et al. 2010 argue that
the step from abnormal data to belief is an instance of abductive
inference, as those who end up endorsing a delusional belief need to
select an explanatory hypothesis for their abnormal data from a range
of relevant hypotheses. Coltheart and colleagues use the Bayesian
model of abductive inference which invites us to ask two questions:
Which hypothesis better explains the data? Which hypothesis is the
most plausible given what we already know? In the case of delusions,
they argue, it is reasonable to adopt the delusional hypothesis given
the data, and the good fit between hypothesis and data swamps general
considerations about the overall implausibility of the
hypothesis. What may not be reasonable is the fact that people with
delusions hang onto their delusions even when they keep gathering
evidence against it—that is, the delusional belief is not
correctly updated in the light of new information. The authors argue
that the information undermining the delusional hypothesis does not
present itself as disconfirming evidence to the person with the
delusion. Thus, the new evidence is interpreted in the light of the
delusion and confabulation is used to fill the gaps. The behaviour of
the person with the delusion is not very different from that of an
obstinate scientist refusing to see the new data as undermining the
support for the theory she proposed and she is now deeply committed

McKay 2012 offers some criticism of the account by Coltheart and
colleagues, and raises the following points among others: (1) it is
not a perfectly rational response to adopt the delusional hypothesis
as an explanation for the abnormal data, unless the probability of the
delusional hypothesis before any abnormal data is gathered is very
high, and this is implausible given the content of some delusional
hypotheses (“my wife has been replaced by an almost identical
impostor”); (2) factor two as described by Coltheart and colleagues
(i.e., a failure to update a belief in the light of conflicting
evidence) cannot precede factor one or be acquired at the same time as
factor one, because such a form of conservatism would prevent the
person from adopting the delusional hypothesis in the first
place. McKay’s positive account is that factor two is not a bias
towards conservatism (which causes new data conflicting with the
delusional hypothesis to be discounted), but a bias towards
explanatory adequacy (which causes the delusional hypothesis
to be adopted and maintained because it fits the abnormal data so
well). McKay also argues that his account is compatible with
prediction-error theories of delusion formation:

An excess of prediction error signal is what underpins
the bias towards explanatory adequacy. Prediction error signals are
triggered by discrepancies between the data expected and the data
encountered. Such signals render salient the unexpected data and
initiate a revision of beliefs to accommodate these data. If there is
an excess of prediction error signal, inappropriately heightened
salience is attached to the data, and belief revision is excessively
accommodatory—biased towards explanatory adequacy. (McKay
2012, p. 18)

The debate is reviewed by Davies and Egan (2013) who helpfully focus
on the distinction between the adoption and the maintenance of the
delusional hypothesis and argue that there is no need to postulate an
additional reason why people hang on to their delusions (a bias
towards conservatism or empirical adequacy). Once the delusional
hypothesis is adopted as an explanation for or an endorsement of the
abnormal data, it is normatively correct from a Bayesian point of view
that it will not be updated or revised for conflicting with previous
beliefs. New evidence would be necessary to prompt an update or a
revision. That said, critical evaluation of the delusional hypothesis
after adoption can still occur if we consider that the delusional
belief was formed as prepotent doxastic response to abnormal data and
thus it is likely to be compartmentalised. In a fragmented belief
system, it could be the case that the compartmentalised belief is
assessed on the basis of previous beliefs.

The debate described above (in a very simplified way) is an
attempt to specify what the second factor is in a two-factor account
of delusion formation. However, as Davies and Egan themselves
acknowledge, the application of idealised models of inference such as
Bayesianism is somehow limited when we are considering actual belief
systems. Even when no pathology is present, biases affecting the
adoption and evaluation of hypotheses are going to be the
(statistical) norm, which makes it difficult to uniquely identify the
problem with the adoption and the persistence of delusional

Although delusions can be irrational to a higher degree than normal
beliefs, as they may be less consistent with a person’s other beliefs
and actions and more resistant to counterevidence, they do not seem to
be irrational in a qualitatively different way from normal
beliefs. This would suggest that they are continuous with irrational
beliefs, although (as we shall see in the next section) there exist
sophisticated philosophical arguments challenging the continuity

4.2 Are delusions beliefs?

According to the doxastic conception of delusions (dominant among
psychologists and psychiatrists), delusions are belief
states—it is an important diagnostic features of delusions that they can lead
to action and that they can be reported with conviction, and
thus that they behave as typical beliefs. (See the entry on
belief.) But there is an increasing
influential view in philosophy warning that the doxastic
characterization of delusions would lead to an oversimplification of
the phenomenon. Although some of the alternative accounts of delusions
(e.g., experiential, phenomenological and metarepresentational) are
critical towards standard doxastic conceptions, they do not necessarily
deny that the phenomenon of delusions involves the formation of normal
or abnormal beliefs. Rather, the central idea seems to be that, even
if people with delusions report false or irrational beliefs, paying
attention only to their first-order cognitive states and to the
doxastic dimension of their pathology can lead to a partial and
incorrect view of the phenomenon of delusions (see also Radden

Some authors emphasize the experiential and
phenomenological character of delusions over the doxastic one
(e.g., Sass 1994; Gold and Hohwy 2000), and others conceive of delusions
not as mere representations of a person’s experienced reality,
but as attitudes towards representations (e.g., Currie 2000; Currie and
Jureidini 2001; Stephens and Graham 2006). Gallagher 2009 argues that
an explanation of the delusion as a mere cognitive error would be
inadequate, and introduces the terminology of delusional
, modes of experience which involve shifts in familiarity
and sense of reality and encompass cognition, bodily changes, affect,
social and environmental factors.

Most of the authors who deny belief status to delusions have a
negative and a positive thesis. The positive thesis is an alternative
account of what delusions are. For instance, one might argue that
delusions are acts of imagination mistakenly taken by a person to have
belief status (Currie and Ravenscroft 2002) or empty speech acts with
no intentional import (Berrios 1991). The negative thesis is an account
of why delusions are not beliefs. Beliefs have certain characteristics,
that is, they are formed and revised on the basis of evidence, they are
consistent with other beliefs, they are action guiding in the relevant
circumstances. If delusions do not share these characteristics, then
they are not beliefs.

Let us list some of the arguments for the negative thesis:

  1. Beliefs are integrated with other
    beliefs. If delusions are not integrated with a person’s beliefs,
    then they are not beliefs.
  2. Beliefs are responsive to evidence. If
    delusions are not responsive to evidence, then they are not
  3. Beliefs guide action. If delusions do not
    guide action, then they are not beliefs.

These arguments are central to the debate about the doxastic nature
of delusions (Bortolotti 2009). For instance, Currie and Jureidini
(2001, p. 161) argue that delusions are more plausibly imaginings than
beliefs, because delusions ‘fail, sometimes spectacularly, to be
integrated with what the subject really does believe’, whereas
there is no requirement that imaginings are consistent with what the
person believes. Berrios (1991) argues that delusions cannot be
beliefs, because, as explanations for an abnormal experience, they are
not regarded even by the person reporting them as more probable than
alternative explanations of the experience. Berrios reaches the
extraordinary conclusions that delusions are not even intentional
states, but utterances without meaning, “empty speech

Assessing arguments in (1) to (3) requires assessing empirical and
conceptual claims. Let’s consider (1), the ‘bad
integration’ objection to the belief status of delusions. In
order to see whether the conclusion is convincing, we need to examine
an empirical claim about delusions first: Do delusions really fail to
integrate with a person’s beliefs? Then, we need to assess a
conceptual claim, the claim that not being integrated with a
person’s beliefs prevents delusions from being beliefs at all. In
many cases, we shall find that the alleged ‘fault’ of
delusions has been exaggerated (e.g., delusions sometimes integrate
well with beliefs), but that it is correct to claim that delusions
exhibit that mark of irrationality (bad integration) to a higher degree
than ordinary beliefs.

The most common versions of anti-doxastic arguments seem to rely on
an idealization of normal belief states, and impose constraints on
delusions that typical beliefs would not meet. The assumption seems to
be that beliefs are essentially rational, and that delusions are not
beliefs because they are not rational. But the abundant psychological
evidence on familiar irrationality tells us that ordinary beliefs are
often irrational in exactly the same way as delusions can be—although
to a lesser degree. It is sufficient to think about hypocrisy,
about prejudiced and superstitious beliefs, and about the many biases
that affect belief updating in normal cognition to realize that the
same kinds of irrationality that we find in delusions are also common
in many ordinary beliefs (e.g., Nisbett and Ross 1980). For the
doxastic conception of delusions, the greatest challenge is to provide
a satisfactory reply to the double-bookkeeping objection: if people
truly believe the content of their delusions, why is their behavior
often inconsistent with it? Aren’t beliefs distinct from other
intentional states in virtue of their action-guiding character? A more
general worry is that the very notion of belief is not theoretically
useful if the criteria for what counts as a belief become too

Independent of any given answer to the question whether delusions
are beliefs, two opposed conceptions of delusions contend the
philosophical scene. One highlights the discontinuity between delusions
and beliefs, and between normal and abnormal cognition, with
consequences for the conceptualization of the disorder, but also for
the availability of therapeutic options, such as cognitive behavioral
therapy, to people with delusions. The other view insists that there is
continuity between delusions and beliefs, and attempts to gather data
both suggesting that people with delusions can reason in much the same
way as people without, and that delusion-like ideas are widespread in
the normal population. Bentall 2003, for instance, gathered a vast
amount of empirical data about the temporal variations in delusions
reported by people affected by psychopathologies, and the presence of
delusion-like beliefs in the normal population.

4.3 Does delusion overlap with self-deception?

There is no consensus on whether self deception and delusion
significantly overlap. Self deception has been traditionally
characterized as driven by motivational factors. Delusions are now
primarily accounted in neurobiological terms, and theories of delusion
formation involve reference to perceptual and cognitive impairments.
However, motivational factors can still play an important role in the
explanation of some delusions, for instance by partially determining
the specific content of the reported delusional state. Thus, one
plausible view is that self-deception and delusion are distinct
phenomena that may overlap in some circumstances (for further analysis,
see Bayne and Fernàndez 2008). There are three arguments for the
view that delusions and self-deception can overlap.

The first view about the relationship between delusions and
self-deception is that, when they overlap, they do so because they both
involve a motivationally biased treatment of evidence. If we agree with
deflationists that the motivationally biased treatment of the evidence
is the key feature of self-deception (Mele 2001 and 2008), then people
with delusions can be said to be self-deceived if they treat the
evidence at their disposal in a motivationally biased way, or if they
search for evidence in a motivationally biased way. This does not seem
to be generally the case, but it is useful to distinguish between
different types of delusions. Some delusions of misidentification (at
least according to neuropsychological accounts) do not seem to be akin
to self-deception, given that there is no fundamental role for
motivational biases in the explanation of how a person comes to hold or
retain the delusion. A different analysis might be appropriate for
other delusions, such as delusions of jealousy or persecution.

The second view is that (some) delusions are extreme cases of
self-deception and that they have a protective and adaptive
function (see Hirstein 2005). An example is offered by Ramachandran,
who discusses anosognosia, the denial of illness, and
somatoparaphrenia, the delusion that a part of one’s body belongs
to someone else. Ramachandran (1996) reports the case of a woman (FD)
who suffered from a right hemisphere stroke which left her with left
hemiplegia. FD could not move without a wheelchair and could not move
her left arm. But when she was asked whether she could walk and she
could engage in activities which require both hands (such as clapping),
she claimed that she could. Ramachandran advances the hypothesis that
behaviors giving rise to confabulations and delusions are an
exaggeration of normal defense mechanisms that have an adaptive
function, as they allow us to create a coherent system of beliefs and
to behave in a stable manner. In normal subjects, the left hemisphere
produces confabulatory explanations aimed at preserving the status
(‘I’m not ill’; ‘My arm can
move’), but the right hemisphere does its job and detects an
anomaly between the hypotheses generated by the left hemisphere and
reality. So, it forces a revision of the belief system. In patients
such as FD, the discrepancy detector no longer works. It is very
plausible that the delusions reported by people with anosognosia
involve motivational aspects. But whether we believe that these
delusions are an exaggerated form of self-deception depends on the
preferred theoretical characterization of self-deception.

The third view about the potential overlap of delusions and
self-deception is that the very existence of delusions (which
shows that doxastic conflict is possible) can help us vindicate the
traditional account of self-deception, according to which a person has
two contradictory beliefs, but she is aware of only one of them,
because she is motivated to remain unaware of the other (McKay et
2005a, p. 314). This account derives from Donald
Davidson’s theory of self-deception (e.g. Davidson 1982 and
1985b). When I deceive myself, I believe a true proposition but act in
such a way as to causing myself to believe the negation of that
proposition. Neil Levy argues that the conditions for self-deception
set by the traditional approach are not necessary for self-deception,
but that the case of FD described by Ramachandran (1996) is living
proof that a person can, at the same time, believe that her arm is
paralyzed, and believe that she can move her arm. Moreover, it is the
belief that her arm is paralyzed that causes her to acquire the belief
that her arm is not. This is Levy’s analysis of the typical
person with anosognosia Levy (2008, p. 234):

  1. Subjects believe that their limb is healthy.
  2. Nevertheless they also have the simultaneous belief (or strong
    suspicion) that their limb is significantly impaired and that they are
    profoundly disturbed by this belief (suspicion).
  3. Condition (1) is satisfied because condition (2) is satisfied;
    that is, subjects are motivated to form the belief that their limb is
    healthy because they have the concurrent belief (suspicion) that it is
    significantly impaired and they are disturbed by this belief

If this analysis is correct, at least one case of delusion (e.g.,
anosognosia) involves doxastic conflict. The most controversial aspect
of this analysis concerns condition (2). Is the belief that their limb
is impaired truly available to people affected by anosognosia? One
could argue that, given that they probably have a deficit in the
discrepancy detector of the right hemisphere of the brain, they have no
awareness of the impairment they deny (see also Hirstein 2005). But
Levy’s reply is that availability comes in degrees. He suggests
that, given that people with paralysis and anosognosia often avoid
tasks that would require mobility when costs for failure are high, and
given that they can acknowledge some difficulties in movement (and say
‘I have arthritis’ or ‘My left arm has always been
weaker’), it is plausible that they have some awareness of their
impairment—although they may lack a fully formed and conscious
belief about it.

The debate about the differences between delusion and
self-deception has centred on whether delusions (just like ordinary
instances of self-deception) are explicable from a folk-psychological
point of view (Bortolotti and Mameli 2012; Murphy 2012, 2013). Murphy
argues that we diagnose delusions when folk psychology runs out of
resources for understanding what someone seems to report as a genuine
belief. By contrast, self-deception does not challenge our
folk-psychological generalisations, because we expect people’s beliefs
to be influenced by their desires at least on some occasions (e.g.,
when stakes are high). Bortolotti and Mameli maintain that the gap
between self-deception and motivated delusions (such as anosognosia)
is narrower than the gap between self-deception and apparently
non-motivated delusions, but even in the latter case folk psychology
can account for the delusional belief as an explanation (irrational as
it may be) of the delusional experience.

In sum, the views summarized here show that it can be very difficult
to justify clear-cut distinctions between delusion and
self-deception. It is diagnostically and scientifically useful to
maintain a distinction between symptoms of conditions such as amnesia,
dementia, or schizophrenia, and the irrational beliefs that
characterize normal cognition, but one should acknowledge that there
are also many elements of genuine overlap.


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