## 1. The Philosophical Significance of Delusion



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, the study
of delusions raises conceptual questions about intentionality,
rationality, agency, and self-knowledge. Moreover, it invites us to
reconsider how perception, cognition, affect, and intentional behavior
interact. 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 neurotypical cognition (see Marshall and
Halligan 1996, pp. 5-6; Langdon and Coltheart 2000, pp.
185-6). Understanding how delusions come about, whether they are
responses to unusual experiences, and whether a reasoning deficit is
responsible for their development can shed light on standard processes
of belief formation. One basic question is what comes first,
perceptual experience or belief (see Campbell 2001). Are delusions
frames of mind that, once adopted, alter our way of seeing the world?
Or do they emerge as explanations for our unusual perceptual
experiences? Does having delusions signal a breakdown of rationality,
and if so, what makes us irrational when we sincerely report a
delusional belief?



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), acceptances (Frankish
2009), and perceptual states (Hohwy and Rajan 2012). Can delusions
qualify as beliefs when they are inconsistent with other things that
we believe? And when they fail to be responsive to the evidence
available to us? Bayne and Pacherie 2005, Bortolotti 2009 and 2012,
and Miyazono 2018 have offered defenses of the doxastic nature of
delusions, whereas other authors have emphasised the differences
between delusions and typical instances of belief. An interesting
position defended by Schwitzgebel 2012 is that delusions are
in-between states, because they match the dispositional profile of
beliefs only in part. 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). More recently, the doxastic nature of
delusions has been further called into question by authors emphasising
the experiential or social nature of delusions (see for instance,
Feyaerts et al. 2021; Bell, Raihani and Wilkinson 2020; Miyazono and
Salice 2021). The nature of delusions is still a hotly debated
issue.



Another strand of investigation in this area is whether the presence
of delusions signals a failure of self knowledge. There are several
apparent manifestations of poor knowledge of the self in delusions
(see Kircher and David 2003; Amador and David 1998). When we are
attributed delusions of passivity, we may not recognize a movement as
our own, and thus have a distorted sense of our personal boundaries;
in thought insertion, we come to believe that there is a thought in
our head that has been placed there by a third party (e.g., Stephens
and Graham 2000). Sometimes we act or feel in a way that is
incompatible with the content of our delusions, or are unable to
endorse the content of our delusion with reasons that are regarded by
others as good reasons (e.g., Gallagher 2009; Bortolotti and Broome
2008, 2009; Fernandez 2010). Finally, the presence of delusions
may make it difficult for us to remember our experienced past
accurately and to plan for the future, because we are unreliable
autobiographers (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 wellbeing. This
literature aims at addressing the conceptualization of delusional
experience and delusional belief 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 pathological beliefs (e.g., Fulford 2004; Bortolotti
2020).



For authors who claim that delusions are pathological, there are at
least six possible (non-exclusive) answers to what makes delusions
pathological:


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 we inhabit a
fictional, non-actual reality and no longer share some fundamental
beliefs and practices with the people around us (for distinct 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.
They defy folk-psychological expectations. This also makes it
difficult for an interpreter to rationalize and understand our
behavior when we report delusions (versions of this idea are explored
in Campbell 2001 and Murphy 2012).
4. Delusions are pathological because (differently from many
nondelusional and yet irrational beliefs) they negatively affect our
wellbeing impairing functioning and causing social isolation
and withdrawal (see Garety and Freeman 1999 for a multidimensional
account of delusions; Bolton 2008 for a harm-related account of mental
illness in general; Pierre 2020 for differences between beliefs in
conspiracy theories and delusions).
5. Delusions are pathological because they have forensic implications,
that is, implications for judgements about whether we can be held
legally accountable for our actions. Hohwy and Rajan 2012 argue that
delusions are attributed to us when others notice significant
impairments in our decision-making and autonomy. Miyazono 2015 claims
that our responsibility-grounding capacities are compromised when we
have delusions.
6. Delusions are pathological because of their etiology. Differently from
other beliefs, they are produced by mechanisms that are dysfunctional
or defective. For instance, their formation process may be
characterized by perceptual aberrations or cognitive deficits (McKay
and Dennett 2009; Coltheart et al. 2010).



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
abduction) 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 Maher
1974, Johns and van Os 2001, Bentall 2003, 2018, and Sullivan-Bissett
2020).



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 apply equally well to more mundane
delusions such as delusions of jealousy or persecution. Moreover, it
is not always obvious that a delusion makes our behavior particularly
difficult to explain or predict. In some cases, delusions can help
make sense of previous adversities or unusual experiences, and even
give meaning to our lives (Gunn and Bortolotti 2018, Ritunnano and
Bortolotti 2021).



The view described in (iv) is very attractive because it captures the
distinction between delusions and other irrational beliefs in terms of
the effects on other aspects of our psychological and social life.
However, using the notions of wellbeing and harm in accounts of
delusions can be problematic. For some people, living with the
delusion may be preferable to living without the delusion: ceasing to
believe that I am a famous TV broadcaster after many years, and
starting to accept that I have been mentally unwell instead, can cause
low self esteem leading me to depression and suicidal thoughts (see
the literature on the *insight* *paradox*, e.g.
Belvederi Murri *et al.* 2016). Indeed, in the philosophical
and psychiatric literature there have been recent explorations of the
idea that some delusions are adaptive in some sense, psychologically,
biologically, and even epistemically (McKay and Dennett 2009, Fineberg
and Corlett 2016, Bortolotti 2016, Ritunnano, Humpston and Broome
2021).



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 are accompanied by severe failures
of autonomous decision-making and give rise to actions for which the
agent is not held accountable, it is not obvious that these cases are
generalisable. Does the mere presence of delusions indicate lack of
autonomy or responsibility? Broome *et al*. (2010) and
Bortolotti *et al*. (2014) discuss case studies raising
pressing issues about the role of delusions in criminal action.
Further, recent literature on whether people with delusions can be
vulnerable to epistemic injustice, suggests that the presence of
delusions does not undermine agency (Sanati and Kyratsous 2015).



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 neurotypical reasoning, and are not
present only in people with delusions. Perceptual anomalies can
explain the formation of *some* delusions, but are not always a
core factor in the formation of *all* delusions. A problem with
hypothesis evaluation may be at the origin of all delusions, but there
is no agreement as to whether the problem is a permanent deficit or a
mere 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 for the way in which our rights and
responsibilities are affected by having delusions. Does having delusions undermine our capacity to consent to treatment or to participate in decision making? 



It is also important to understand whether we can be regarded as
morally responsible for our actions if we engage in morally
problematic behaviour or commit acts of violence or other crimes that
are motivated by our delusions. As a
consequence of the failures of rationality and self knowledge that
characterize some delusions, we
may appear as if we are 'in two minds', and we may not
always present ourselves as unified agents with a coherent set of
beliefs and preferences (e.g., Kennett and Matthews 2009). As a
result, we may be judged to be (locally or temporally) unable to
exercise our capacity for autonomous thought and action. 



Another important question concerns the possibility that we are
vulnerable to epistemic injustice when we report unusual beliefs that
are regarded as delusional and symptomatic of mental distress.
Epistemic injustice occurs when our contribution to
knowledge is dismissed due to negative stereotypes associated with
aspects of our identity (e.g., "Women are bad at maths"
or "Black people have lower IQ than white people"). In the case of
delusions, a risk is that other people come to dismiss our
contribution to a conversation or come to consider us as lacking
authority or credibility based on the fact that some of our beliefs
are delusional (Sanati and Kyratsous 2015). But the presence of
delusions does not necessarily speak against our overall capacity
to produce and share knowledge, so dismissing our contribution due to
our having some delusional beliefs may be unfair.


## 2. The Nature of Delusion



How are delusions defined?


### 2.1 Defining delusion



Definitions of delusions are based on the behavior we exhibit
when experiencing a delusion rather than on the underlying mechanisms
responsible for our behavior. The behavior associated with having delusions
is often described in epistemic terms, using concepts such as belief,
truth, rationality, evidence, and justification. For instance,
delusions may be defined as *beliefs* held
with *conviction* in spite of having little *empirical
support* (where the epistemic concepts are in italics). 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 contrary:



> 
> 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 raised a
number of questions about the DSM definition. According to the
challenges, the DSM definition of delusions cannot capture what is
distinctive about delusions, and does not tell us 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. Exemplars 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. In the passage below, 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. 1043)
> 



The *Diagnostic and Statistical Manual of Mental Disorders* has
been updated through the years 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
delimiting 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 our wellbeing. Some other definitions of delusion make
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 our lives.


### 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



When we have persecutory delusions, we believe that we are treated
with hostility, and that harm is coming our way. When we have
delusions of mirrored-self misidentification, we usually preserve the
capacity to recognize images in the mirror as reflections, but do not
recognize our own face reflected in the mirror. So, we come to think
that there is a person in the mirror, a stranger who looks very much
like we do. In either case, the delusion is resistant to
counterevidence and can have pervasive effects on our lives. One of
the differences is that persecutory delusions are
*polythematic* whereas delusions of mirrored-self
misidentification are *monothematic.*



Polythematic delusions extend to more than one theme, where the themes
can be interrelated. Delusions of persecution are very common
polythematic delusions. We may believe to be surrounded by alien
forces controlling our actions, where the alien forces are slowly
taking over people's bodies including our own. In that case, we
have delusions of persecution and delusions of alien control.
Typically, these delusions are interrelated and are manifest in the
interpretation of most events occurring in our lives. Other examples
of delusions that can be polythematic are the belief that we are
geniuses but are misunderstood and underestimated by others
(grandeur), and the belief that we are secretly loved by a famous or
powerful person (erotomania).



Delusions of mirrored-self misidentification, instead, are
*monothematic.* Apart from the content of delusion itself, we
may report no other unusual belief. Thus, when we systematically fail
to recognize our image in the mirror and come to think that there is a
person identical to us on the other side, and we have no other unusual
belief, we have a monothematic delusion. Other examples of delusions
that are usually monothematic and are discussed 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
we are disembodied or dead. 


#### 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 a more detailed explanation and some examples). The
distinction between circumscribed and elaborated delusions is relevant
to the level of integration between our delusions and our other
intentional states and to the extent to which our 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 
behavior. For instance, a man
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 we form other beliefs that revolve around
the theme of the delusion. For instance, a woman with Capgras can
develop paranoid thoughts related to the content of her delusion,
along the lines that the person impersonating her husband has evil intentions and will cause her
harm when the occasion presents itself.


#### 2.2.4 Primary versus secondary



Depending on whether the delusion has some understandable origin and
we can defend it with arguments, it 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 is
problematic, because it is difficult to establish whether there are
antecedents of the delusion, and
for other methodological and clinical reasons (e.g., Miller and Karoni
1996, p. 489). A delusion may arise out of our
previous adverse experience that is not known to other, and some studies show
that even very unusual beliefs can be made sense of in the context of our lives (Gunn and Larkin 2020). 



New readings of the distinction between primary and secondary have
been provided in the philosophical literature on delusions, where
distinctions are made 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; Aimola Davies and Davies 2009 distinguish between
*pathologies of belief* and *pathological beliefs* on
similar lines).


## 3. Theoretical Approaches to Delusion



There are several theoretical approaches to delusion formation which
attempt to explain the surface features of delusions by reference to
unusual experiences, reasoning biases, neuropsychological deficits,
motivational factors, and prediction errors, 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 mostly 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, no
neurobiological deficits are needed in the explanation of how delusions are
formed. Rather, delusions are caused by motivational factors. For
instance, delusions of persecution would be developed in order to
protect us from low self-esteem and depression, and would be due to
the attribution of negative events to some malevolent other rather
than to ourselves. 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 Capgras, affecting the
face recognition system. For delusions that are supposed to play a
defensive or self-enhancing role (e.g., persecution, anosognosia, and
erotomania), psychodynamic accounts or hybrid accounts featuring a
combination of neurobiological and motivational factors are 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 its psychological manifestations, culminating with 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 romantic partner
is still faithful to us 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 our partner would cause (see
example in Butler 2000 as cited and discussed by McKay *et al.*
2005a, p. 313). 


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



According to popular neuropsychological accounts, delusions are the
result of an unusual perceptual experience (Maher 1974); an unusual
experience accompanied by reasoning biases (Garety and Freeman 1999;
Garety *et al*. 2001; McKay 2012); a breakdown of certain
aspects of perception and cognition including a deficit in hypothesis
or belief evaluation (Langdon and Coltheart 2000, Coltheart *et
al.* 2010); or a failure of predictive coding (Fletcher and Frith
2009; Corlett *et al*. 2010). 



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 at first to
articulate a delusional hypothesis to explain unusual experience, it
is not rational to maintain the delusional hypothesis in the face of
counterevidence. Recent defences of the one-factor theory have
emphasised that cognitive features in people who develop delusional
beliefs are within the normal range and there is no need to postulate
any reasoning deficit to explain delusion formation (Sullivan-Bissett
2018, 2020).



For two-factor theorists (Davies *et al*. 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
reasoning deficits. Developments of this theory have been offered by
Aimola Davies and Davies 2009, by Coltheart *et al*. 2010, by
McKay 2012, and by Davies and Egan 2013.



In the two-factor theory framework, an unusual 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 (factor 1), 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
unusual event (due to a reduction of autonomic response) is not the
only factor responsible for the formation of the Capgras 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 unusual
event, a factor 2 is required, and this can be 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). In a more recent version of the two-factor
model, the problem is no longer identified with the adoption of the
delusional belief, which is described as the best explanation for the
unusual event. Rather, the problem lies with failing to give up the
delusional belief in the face of overwhelming counterevidence after
its adoption (Coltheart *et al.* 2010).



In another version of the two-factor theory (McKay 2012), the young
woman adopts the impostor hypothesis because she suffers from a
neuropsychological impairment responsible for the unusual event
(factor 1) resulting in the anomalous experience. But the unusual
event is not the only factor responsible for the formation of the
delusion. The young woman also has a bias towards explanatory adequacy
(factor 2) which leads her to accept a hypothesis that matches her
unusual experience even when such hypothesis has a low prior
probability and conflicts with some of her existing beliefs. The
Capgras delusion is adopted because the delusional hypothesis explains
('fits') the data, that is, the absence of a feeling of
familiarity towards a face that should be well-known.



According to a delusion formation theory inspired by predictive
coding, we form expectations about our experience in line with a
general model of how the world is. Greater attention is paid to those
events which defy expectations, as they might require a revision of
our model of the world, the one we have adopted up to that point. The
discrepancy between what we expect and the information we take in is
an important part of the way learning occurs. When our expectations
are not met, a prediction error is coded, and our model 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 us to update our current beliefs
about reality, which happen to be correct. The woman who sees 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 an attempt to explain the unexpected event
('This is not my father!'). This results in the formation
of a delusion (Corlett *et al*. 2007) where the formation of
the delusion is principally an "error of learning" (Fine and Corlett
2016).



Some hybrid explanations of delusion formation have been proposed (see
e.g. Galbraith 2014), for instance combining insights from the
two-factor theory and the predictive coding account (e.g., Miyazono
and McKay 2019).


### 3.3 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. Our 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 al*.
2001). For instance, I experience people watching me with suspicion or
hostility, and as a result I form the hypothesis that they intend to
cause me harm; 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 them.



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 delusional belief *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 young woman's experience would be that
of someone looking very much like her father but not being her father.
The delusion would be an explanation of the fact that the man looks
like her father, but his face feels strange to her: the man must be an
impostor. In persecution, the experience would be that of people
behaving with hostility towards us, and the delusion would be an
explanation why they behave with hostility: they intend to cause us
harm. This account leaves it open that the same experience could be
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; Bongiorno 2019), 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, a way of taking the experience
at face value: the content of the experience is taken as veridical and
believed. In Capgras, the young woman's experience is that of a
man looking very much like her father but being an impostor, and when
the experience is endorsed, it becomes the delusional belief that her
father has been replaced by an impostor. In persecution, our experience
is that of people intending to cause us harm, and when it is endorsed,
it becomes the delusional belief that those people intend to cause us
harm.



There are further issues relevant to the debate between explanationism
and endorsement theory, including whether for the person with the
delusion the unusual experience needs to be conscious and whether
there needs to be an inferential step between experience and belief
(Bongiorno and Bortolotti 2019).


## 4. Delusions and the Continuity Thesis



This section focuses on four 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 our delusions are
markedly different from other beliefs we have and whether they can be
described as pathological in some sense.


### 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
irrational? If so, in what way?



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 unusual nature of the delusion is due to the unusual nature 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 unusual
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 unusual
experiences and do form the delusion are instead affected by an
additional problem, maybe 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 unusual nature
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 associated with having
delusions is overall rational. An explanation is needed for 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 we
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*.



So, one argument for the view that delusions are irrational comes from
an examination of 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: when we have delusions, we may ignore
relevant evidence or attempt to defend our delusional beliefs from
apparent objections with obvious confabulations. Often these attempts
are deeply perplexing, and the reasons offered for believing the
content of our delusions do not appear as be good reasons to others:
in one of the examples of delusions offered at the start of this
entry, a woman incorrectly believed that a man was in love with her
and her evidence for that claim was that he was sending her secret
love messages hidden in the license plates on cars of a certain state.
However, the notion that delusions fail to be responsive to evidence
has also been challenged: for instance, Flores 2021 convincingly
argues that when we have delusions we have the capacity to respond to
evidence that has a bearing on the content of the delusions, but other
factors contribute to the fact that we resist counterevidence, such as
motivational factors.



One reason why maintaining delusional beliefs may be seen as irrational
is that the content of the delusions is often in tension with the
content of our other beliefs, or with our behavior (Gerrans 2000, p.
114 and Young 2000, p. 49) . That is, when a delusion is introduced
and maintained within our belief system, it creates inconsistencies
and does not always drive the actions we would expect given its
content. Here are some examples. 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 our
apparent strong conviction in the content of the delusion be squared
with our failure to act on it? One hypothesis is that the content of
the delusion is not genuinely believed. Another hypothesis is that we
genuinely believe the content of the delusion but do not convert it
into action, because we fail to acquire or maintain the motivation to
act on the delusion. This would be consistent with negative symptoms
of schizophrenia, as Bortolotti and Broome 2012 argue. 



One should not be too impressed by evidence of 'behavioural
inertia', however, as there are also
many examples in the literature of delusions driving action.
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 and as a result 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).



Relevant to the question whether delusions are rational, the empirical
literature suggests that our reasoning performance when we have
delusions reflects data-gathering and attribution biases. For
instance, it has been argued that people with delusions 'jump to
conclusions'; 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) whereas 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).
People with delusions seem to be worse than controls at inhibiting the
evidence of their senses when it conflicts with other things they know
(Langdon *et al*. 2008b) and 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 that we can 
deviate from statistically normal reasoning performance when we have
delusions.



A recent debate relevant to the rationality of delusions concerns the
step from unusual events to delusional beliefs. (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 unusual 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
unusual 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 that the new data
undermines the support for the theory she is deeply committed to.



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 unusual data, unless the probability of the
delusional hypothesis before the unusual 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 we hang on to our delusions (either a bias
towards conservatism or empirical adequacy). Once the delusional
hypothesis is adopted as an explanation for or an endorsement of the
unusual data, it is correct from a Bayesian point of view that it will
not be updated or revised due to a conflict with previous beliefs. New
evidence would be necessary to prompt an update or a revision. That
said, a critical evaluation of the maintenance of the delusional
hypothesis can still be made because the delusional belief was formed
as prepotent doxastic response to unusual data and thus it is likely
to be compartmentalised. As Davies and Egan themselves acknowledge,
the application of idealised models of inference such as Bayesianism
is somehow limited in the examination of 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 beliefs.



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


### 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 increasingly 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 beliefs. Rather, the central idea
seems to be that, even if we report false or irrational beliefs when
we experience delusions, paying attention only to our first-order
cognitive states and to the doxastic dimension of our mental life can
lead to a partial and incorrect view of the phenomenon of delusions
(see also Radden 2010).



Some authors emphasize the *experiential* and
*phenomenological* character of delusions over the doxastic one
(e.g., Sass 1994; Gold and Hohwy 2000; Feyaerts et al. 2021), and
others conceive of delusions not as mere representations of our
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 realities*, 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 our beliefs, then they are not
beliefs.
2. Beliefs are responsive to evidence. If delusions are not
responsive to evidence, then they are not beliefs.
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 beliefs. Berrios 1991 argues that delusions cannot be beliefs for us,
because we do not regard them as the most probable explanations for our unusual experience. Berrios reaches the
extraordinary conclusions that delusions are not even intentional
states, but utterances without meaning, "empty speech
acts".



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, an empirical claim
about delusions needs to be examined first: Do delusions really fail
to integrate with our beliefs? Then, a conceptual claim needs
assessing, the claim that not being integrated with our beliefs
prevents delusions from being beliefs themselves. In many cases, the
alleged 'fault' of delusions is exaggerated (e.g.,
delusions sometimes integrate well with our beliefs), but it would be plausible to claim that delusions exhibit that feature (bad integration)
to a higher degree than nondelusional beliefs.



The most common versions of anti-doxastic arguments seem to rely on an
idealization of belief states, and impose constraints on
delusions that not even typical beliefs would 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
nondelusional 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 nondelusional beliefs (e.g., Nisbett
and Ross 1980; more recently Bentall 2018). A general worry is that
the very notion of belief is not theoretically useful if the criteria
for what counts as a belief become too loose.



Independent of any given answer to the question whether delusions are
beliefs, two conceptions of delusions divide the philosophical scene.
One highlights the discontinuity between delusions and beliefs, and
between neurotypical and neurodivergent cognition, with various
consequences for what counts as pathological, but also for the
availability of therapeutic options, such as cognitive behavioral
therapy, for conditions characterised by delusions. The other view
insists that there is continuity between delusions and beliefs, and
attempts to gather data both suggesting that people with delusions
reason in much the same way as people without, and that delusion-like
ideas are widespread in the nonclinical population. Some studies have
gathered a vast amount of empirical data about the temporal variations
in delusions in context of psychopathologies, and the
presence of delusion-like beliefs in the nonclinical population (e.g.,
Bentall 2003).


### 4.3 Do delusions overlap with self-deception or beliefs in conspiracy theories?



There is no consensus on whether delusions overlap with instances of
belief that are frequent in the nonclinical population, such as self
deception or beliefs in conspiracy theories. Philosophers who ask
such questions may be interested in supporting or challenging the
continuity theory; mapping reasons why delusions should (or should
not) be regarded as pathological; or wanting to clarify the
distinctiveness of the phenomenon of clinical delusions.



Self deception has been traditionally characterized as driven by
motivational factors whereas delusions are now primarily accounted in
neuropsychological terms. Although theories of delusion formation
involve reference to perceptual and cognitive impairments, as we saw,
motivational factors play an important role in the explanation of some
delusions, e.g. 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
Fernandez 2008). There are three arguments for the view that
delusions and self-deception 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 a
motivationally biased treatment of evidence is the key feature of
self-deception (as for deflationists such as Mele 2001 and 2008), then
when we have delusions we can be said to be self-deceived if we treat
the evidence at our disposal or search for evidence in a
motivationally biased way. This does not seem to be generally the case
with delusions, but can apply to some cases 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 we come to hold or retain the delusion. A different
analysis might be appropriate for 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 the neurotypical case, the left hemisphere produces
confabulatory explanations aimed at preserving the *status quo*
('I'm not ill'; 'My arm can move'), but
the right hemisphere 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 delusions in anosognosia
involve motivational aspects. But whether 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 this view, we have
two contradictory beliefs, but we are aware of only one of them,
because we are motivated to remain unaware of the other (McKay *et
al.* 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. FD believes that her limb is healthy.
2. Nevertheless she also has the simultaneous belief (or strong
suspicion) that her limb is significantly impaired and she is
profoundly disturbed by this belief (suspicion).
3. Condition (1) is satisfied because condition (2) is satisfied;
that is, FD is motivated to form the belief that her limb is healthy
because she has the concurrent belief (suspicion) that it is
significantly impaired and she is disturbed by this belief
(suspicion).



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 her limb
is impaired truly available to FD? One could argue that, given that FD
probably has a deficit in the discrepancy detector of the right
hemisphere of the brain, she has no awareness of the impairment she
denies (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
delusions are diagnosed when folk psychology runs out of resources for
understanding what we seem to report as a genuine belief. By contrast,
self-deception does not challenge our folk-psychological
generalisations, because our beliefs are expected to be influenced by
our 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 nonmotivated delusions, but
even in the latter case folk psychology can account for the delusional
belief as an explanation (imperfect as it may be) of the delusional
experience.



In recent commentaries on the spread of conspiracy theories during a
crisis, comparisons between beliefs in conspiracy theories and
delusions have been ubiquitous (Bortolotti et al. 2021). For instance,
people have argued that what makes beliefs in flat Earth and climate
denial problematic is not their failing to represent reality
accurately, but their rigidity and incorrigibility, which they see as
a typical feature of delusional beliefs. The comparison has been made
on the basis of three criteria: (a) how the beliefs manifest (e.g.,
implausibility and unshakeability); (b) how the beliefs are formed
(e.g., faulty monitoring of ideas); and (c) how the beliefs affect
wellbeing and good functioning.



Let's consider the criteria in turn. With respect to surface
features, both clinical delusions and beliefs in conspiracy theories
appear implausible to those who do not endorse them, and in some
circumstances quite unusual and bizarre as well. It is their alleged
epistemic irrationality, though, that seems to play the central role
in the comparison. It is commonly assumed that clinical delusions and
beliefs in conspiracy theories are neither well supported by evidence
nor responsive to counterevidence. As we saw, there is a variation in
these features when they are applied to delusions, and similarly when
they are applied to beliefs in conspiracy theories. In the case of
delusions, the unusual experience we have may be considered prima
facie evidence for the delusional hypothesis; and in the case of
conspiracy theories, if we belong to a group that has been
marginalised and excluded, then our previous experience with the
authorities may legitimise our mistrusting them as sources of
information (Levy 2019).



With respect to how the beliefs are formed, there is no consensus.
Some argue that the same models used to describe delusion formation
can be invoked in the description of how beliefs in conspiracy
theories emerge. For instance, for two-factor theorists, there is a
first factor which is unusual experience in the case of delusion and
epistemic mistrust in the case of beliefs in conspiracy theories.
Then, there is a common second factor which comprises some reasoning
biases and motivational factors (e.g., Pierre 2020). The first factor
would explain the theme of the delusion or conspiracy. The second
factor would explain why the belief is endorsed and maintained despite
its *prima facie* implausibility. For researchers working
within a predictive coding account, paranoia predicts difficulty in
updating beliefs across the clinical-nonclinical divide, which
suggests that some of the same mechanisms may be responsible for how
people respond to uncertainties (Reed et al. 2020). Other researchers
though deny that there is overlap in etiology, and point to the fact
that no cognitive dysfunction needs to be postulated to explain the
formation of beliefs in conspiracy theories (e.g., Oliver and Wood
2014). 



Finally, with respect to the downstream effects of endorsing
delusional beliefs and beliefs in conspiracy theories, differences are marked. Whereas delusions are often
characterised by disruption in psychological functioning and social
withdrawal and isolation, beliefs in conspiracy theories can be seen
as cementing some relationships in well-defined social groups and as
being protective against anxiety and stress. A closer examination may
shed some doubt on the differences though. Some delusions can be
adaptive in the short-term and some beliefs in conspiracy theories can
be harmful in the long term.



In sum, the views summarized here show that it can be very difficult
to justify clear-cut distinctions between delusion and self-deception,
and between delusions and beliefs in conspiracy theories. It is
diagnostically and scientifically useful to maintain a distinction
between symptoms of clinical conditions and manifestations of everyday
irrationality, but the elements of genuine overlap should also be
acknowledged.