Developing general models and theories of addiction

Transcription

1 Developing general models and theories of addiction Robert West Department of Behavioural Science and Health, University College London Simon Christmas Independent consultant Janna Hastings Babraham Institute, University of Cambridge Susan Michie Centre for Behaviour Change, University College London Introduction The science of addiction is being hampered by confusion in concepts and terms, and a multiplicity of models and theoretical approaches that make little reference to each other. In this respect it has much in common with other areas of social, clinical and behavioural sciences. Technologies now exist and are being rapidly advanced that can address this problem, and other sciences are already making use of them. In particular, what are known as ontologies (as used in computer science) and the Semantic Web could revolutionise our ability to formulate models and theories in addiction which can then provide much needed direction to the scientific endeavour. The field of biology suffered from a similar problem until the development of what is known as the Gene Ontology (Ashburner et al., 2000). The gene ontology is not just about genes, but is a representational system for the whole of biology, unifying terms, definitions and models across species and research groups in a way that has revolutionised the field (Lewis, 2017). This chapter introduces readers to ontologies and the semantic web, and explores their potential use in developing and expressing models and theories of addiction in ways that allow relationships to be examined between them, and between these and more general models and theories in clinical, General theory of addiction Page 1

2 population and behavioural sciences. These technologies also allow investigation of construct relationships within models, necessary for testing and hence refining and advancing them. We begin by describing a central challenge facing the study of addiction: the need to achieve clarity of constructs and develop consensus while at the same time recognising that divergent views have utility. We then move on to describe some key characteristics of the Semantic Web, and the ways in which these provide a pragmatic way of responding to this challenge. We conclude by looking at the potential value of existing ontologies in developing a general theory of addiction. The challenge: clarity and diversity Models of addiction are necessary for building addiction science and developing effective interventions to combat this problem. If they go beyond describing observed relationships (descriptive models such as tobacco price elasticity (Gallus, Schiaffino, La Vecchia, Townsend, & Fernandez, 2006)) and attempt to explain phenomena we refer to them as theories (e.g. the dopamine theory of drug reward (Blum et al., 2015)). There are a plethora of models and theories of addiction differing in scope, emphasis, constructs and propositions but they have not been expressed in ways that allow them to be compared, tested or integrated. The term addiction is itself an example of this lack of clarity in relation to key constructs. Many cases of psychoactive drug use have features that lead to the users being labelled as suffering from a condition called addiction. Sets of such features have been listed in diagnostic criteria, such as DSM-5 (American Psychiatric Association, 2013) and ICD-10 (World Health Organisation, 2016). These include continued use despite harmful consequences, experience of adverse withdrawal symptoms during periods of non-use, difficulties controlling use, high levels of use, and repeated strong motivation to use. Features can be present to different degrees, which means that thresholds are required for deciding whether or not a pattern of drug use is addictive. These thresholds are to a large extent arbitrary and context dependent. An alternative, rather than considering addiction to be present or absent depending on whether some threshold is exceeded, is to assess the degree to which the features are evident to specify a degree of addictedness or severity of addiction (Gossop et al., 1995). With multiple features potentially being involved, different ways of characterising these features, and different thresholds potentially being applicable, there can be substantial differences of view in whether an individual s pattern of drug use is considered addictive, or the degree of addictedness. These differences mean that issues such as the prevalence of addictions and theories concerning the causes of addiction are subject to differing viewpoints that cannot be reconciled solely by reference to objectively determined facts. The picture is further complicated by the fact that different psychoactive drugs have different patterns of use and show different degrees and patterns of addictive features, as do different ways of using the same drug. An additional complication is that behaviours that do not involve psychoactive drugs, such as gambling, can show similar characteristics to drug addiction and so the concept of addiction appears to apply to those behaviours as well. A similar lack of clarity exists for many other constructs which are important to a scientific understanding of addiction. Any general model or theory of addiction would need to capture processes that have been identified as important in its development and maintenance (Orford, 2001; West & Brown, 2013) (Table 1). Although conceptually different, many of these processes have features in common, or else the distinction between them is nuanced. For example, incentive sensitisation and drive theory both involve craving. In the case of incentive sensitisation, cravings are General theory of addiction Page 2

3 generated in response to cues through repeated exposure enhancing a direct link between those cues and the experience of wanting whereas drive theory proposes that cravings are generated through exposure to a drug leading to a state that is relieved by taking the drug. Some models and theories are fundamentally neurophysiological whereas others focus on social processes. Table 1: Processes commonly included in models of addiction (West, 2017) Processes Description Propositions in existing models Cost-benefit analysis Incentive sensitisation Reward seeking Attachment Drive reduction Distress avoidance Social influence Impaired control Classical conditioning The benefits of the addictive behaviour are judged by the addict to outweigh the costs Repeated exposure to addictive drugs leads to sensitisation of brain pathways that generate feelings of wanting in response to drug cues independent of feelings of liking Addicts learn that addictive behaviours provide positive feelings of enjoyment and euphoria Addicts become emotionally attached to drugs or addictive behaviours because these have been reliable sources of comfort or gratification Repeated engagement in an addictive behaviour results in development of an acquired drive, which is experienced as craving, after a period of abstinence. Addicts learn that addictive behaviours relieve mental and physical distress caused by mental health problems, life circumstances, and/or withdrawal symptoms Cultural, sub-cultural, peer group and/or family norms promote or are permissive of addictive behaviour Addictive behaviours develop, and are maintained by, pre-existing or acquired inefficiencies in brain systems required for impulse control Repeated pairing of stimuli (cues) associated with effects of addictive behaviours leads those stimuli to At least some people addicted to alcohol believe that the benefits in terms of anxiety relief and mental escape are worth the financial, social and health costs Smoking crack cocaine leads to feelings of craving in situations similar to those where this has occurred, independent of feelings of euphoria produced by the drug Methamphetamine users seek the rush provided by the drug Smokers often report feeling a sense of bereavement during the early stages of stopping smoking: like they have lost a cherished friend or family member Repeated use of nicotine alters brain physiology so that abstinence results in an acquired drive state, experienced as craving Repeated use of heroin results in changes in brain chemistry leading to adverse mood and physical symptoms when concentrations of the drug fall below certain concentrations in the brain Family and peer group are important factors influencing the development of smoking and alcohol consumption Use of stimulant drugs leads to impairment in frontal lobe functioning required to inhibit impulses to continually repeat the use leading to bingeing Lights, images and sounds are used by gaming machine General theory of addiction Page 3

4 Operant conditioning generate anticipatory reactions to those effects Addictive behaviour are followed by powerful positive or negative reinforcers (rewards or offset of aversive stimuli) in the presence of discriminative stimuli (cues), so that those stimuli come to provoke a strong impulse to engage in the behaviour manufacturers to promote high rates of use of those machines Use of psycho-stimulants is maintained by the positive reinforcing properties of these drugs None of the processes believed to underlie addiction are limited to addictive behaviours; they are all involved in the development of other motivations (West & Brown, 2013). What makes a behaviour pattern addictive is the strength of the motivational forces generated and/or the way that particular drugs or behaviours interact with the motivational system to create a positive feedback cycle rather than the self-correcting systems that normally operate to ensure that no one behaviour receives an unwarranted priority at the expense of others. For example, with palatable food, our natural processes of satiation and habituation reduce its rewarding value as we eat more of it during a meal. By contrast, with cocaine and amphetamine, reward mechanisms become sensitised to the impulse generating effects of the drug (Berridge & Robinson, 2016). In the case of alcohol dependence, addicts develop craving, rebound anxiety and adverse physical symptoms which provide a very powerful motivation to resume drinking (Seo & Sinha, 2014). In the case of tobacco smoking, rapid ingestion of nicotine leads to strong cravings through multiple mechanisms, including creation of an acquired drive state, similar to hunger, when CNS nicotine concentrations become depleted (West, 2009). So development of general models and theories of addiction will involve bringing together a wide range of constructs and processes relating to canonical motivational theory (i.e. a theory of what normally happens) and how abnormalities in motivation occur. These processes will be both internal and external to individuals, groups and populations. Diversity in the study of addiction While there is a need for greater clarity of constructs in the study of addiction, no investigator or organisation has the authority, or expertise, to propose a single unifying conceptual framework. Figure 1: du Plessis classification of addiction theories (du Plessis, 2014) a) Eight major methodologies b) Types of addiction theory classified according to the eight methodologies General theory of addiction Page 4

5 A general theory of addiction has yet to be developed, but a key requirement for such a theory is that it should recognise and accommodate multiple viewpoints on addiction, and not be limited to a single viewpoint such as the medical model (construing addiction in term of a mental disorder, disease or disease process). Figure 1 shows du Plessis classification of the types of theory that would need to be recognised and accommodated by a general theory of addiction according to eight major methodologies (du Plessis, 2014). Some specific theories and several theoretical approaches have attempted to span a number of the zones in this classification. Of the specific theories, the theory of Excessive Appetites and PRIME Theory have been elaborated in some detail (Orford, 2001; West & Brown, 2013), respectively). It is important not to overstate this diversity. Despite the complexities discussed earlier, the observed phenomena that are captured by a term like addiction are important, and it is useful to give a label to a construct that captures these. Moreover, there are shared understandings of many of the features that characterise addictive drug use, even where there may be disagreement around details and where emphasis should be placed. On the other hand, it should go without saying that a general theory which achieved greater clarity by simply ignoring this diversity of viewpoints would not in fact be a general theory at all. To move forward, we need a way to achieve clarity of constructs and develop consensus, while at the same time recognising that divergent views have utility. Addressing the challenge in the Semantic Web The technology associated with the Semantic Web provides the basis for a response to the challenge we have outlined: a way for the science of addiction to move from its current state of confused, confusing and imprecise terminology to a way of representing addiction and models of addiction that will promote collaborative working, respect and preserve different viewpoints on addiction, but nevertheless allow all research results to be integrated, and theoretical predictions to be tested, and so advance the field. What is the Semantic Web? One way to answer this question is to chart the development of the Semantic Web from the Worldwide Web (www). The Worldwide Web has revolutionised our lives by making information available from a vast range of sources. It defines a technological framework for locating and exchanging diverse content types for example, text, images, and films. At the heart of this framework is the Uniform Resource Locator (URL), a unique address for each of the billions of different web pages. These URLs are stored as a code that means nothing to human readers, but a General theory of addiction Page 5

6 system of domain names has been developed to link them to a name that humans can read and understand. These also have to be unique. Thus is a domain name for the journal, Addiction, that uniquely points to the journal s home page. Web pages use URLs to link to other pages to create the Worldwide Web. The www has a huge limitation, however. The information contained in web pages is designed for humans to consume, and is difficult for machines to interpret, which hinders the ability of software to help in the discovery, synthesis, and summary of online content. Google, Yahoo and other search engines make their developers billions of dollars by trying to address this limitation, but for the most part all that search engines are able to do is to point to web pages that might contain the information you want; finding the right pages can often require multiple attempts to formulate a search in the right way. For example, a search for Addiction conferences will not return results from pages entitled Addiction meetings ; a search for addiction will not return results from pages entitled dependence. There is no representation of the underlying meaning of content which is sensitive to synonyms or closely related constructs, only the superficial textual representation of the content encoded directly in the web pages. The Semantic Web is a radical advance on this. Instead of only making content available in text on web pages, the Semantic Web allows individual pieces of information within those pages to be assigned a meaning and a specific location. Rather than only having a string of text to interpret ( addiction / dependence ), within the Semantic Web units of information are explicitly defined in shared vocabularies or ontologies computational representations of knowledge in a particular domain. Ontologies link together the words, expressions and language which humans use to refer to things, with computable formal definitions of those things that allow computers to distinguish one type of thing from another. The Semantic Web is implemented in the most general sense as a collection of statements which take the form subject-predicate-object, known as triples. This very generic form of representation is then populated with entities (subjects and objects) and relationships drawn from shared, welldefined vocabularies in order to be clear on their interpretation and meaning. Units of information are assigned Uniform Resource Identifiers (URIs), each of which points to a unique entity, concept or resource that has an address on the web, and each URI is then used in triples represented in what is known as the Resource Description Framework (RDF). Computers are thus given the ability to understand terms such as psychological condition by how such a term is related to other terms set out in a network of relationships in RDF triples and definitions within ontologies. This forces the definitions of all terms used within the Semantic Web to be explicit and distinguishable. Humans, by contrast, understand terms in ways that combine formal definitions with natural language meanings, which creates ambiguity. Although nuances of natural language provide a richness that is helpful in certain circumstances, ambiguities impede the scientific testing and advance of ideas. To make content computationally interpretable within the Semantic Web forces the use of clear and distinguishable representations for each different sort of thing, which explicitly addresses the sorts of confusion that are hindering progress in developing the science of addiction. For example, Table 2 shows examples of propositions about addiction expressed in the form of triples. These provide a way of explicitly expressing models of addiction that allow different models General theory of addiction Page 6

7 to be compared and related to more general models of behaviour. Setting out each proposition in a model of addiction in this way allows that proposition to be examined and tested. Table 2: Examples of propositions about addiction in the form of triples expressed as an approximation to natural language addiction is a subclass of mental disorder has subclass substance addiction has subclass behavioural addiction inheres in people has manifestation addictive behaviour has attribute addiction duration has attribute addiction strength has attribute addiction harmfulness has attribute addiction severity has part strong motivation has part significant harm potential has manifestation repeated motivation episodes A considerable amount of unproductive debate in the field of addiction, as in many other areas of social, clinical and behavioural science, arises because the individual propositions within theories cannot be separated out for scrutiny. Thus different researchers may take differing views on any or all of the propositions in Table 2, but the fact that they are set out in this way allows for discussion about the merits and demerits of each one to be analysed systematically, whether by reference to evidence or inference. Diversity of perspectives in the Semantic Web The way in which the Semantic Web works, and in particular the fact that the elements from which RDF triples are composed are explicitly assigned meaning in ontologies and by reference to their links to other elements as stated in other triples, ensures clarity of constructs. At the same time, the Semantic Web also provides a way of accommodating diversity. Indeed, a founding principle of the Semantic Web, just like the Worldwide Web, is that anyone can say anything about anything. For example, different people can continue to use terms such as addiction differently, since these terms will be assigned separate URIs and definitions despite sharing the same superficial label. In fact, there is a built in way to represent different perspectives on the Semantic Web through use of different namespaces. Stated simply, if I want to define addiction in a new way I can simply create my new meaning of addiction in my own unique namespace, and explicitly refer to that version of addiction in my content. This makes it completely clear what the term means as I am using it. Importantly it also allows for systematic comparisons between my usage and someone else s. In principle the use of namespaces could result in anarchy if no-one agreed about the definitions of anything. But this is no different from what happens with natural language. The difference in the case of the Semantic Web and ontologies is that disagreements in how terms are used are explicit and defined. They can then be subject to scrutiny, and differences potentially resolved where such resolution is useful. In practice, many researchers do agree on key definitions and can therefore use a single shared namespace to reflect this consensus. This is directly analogous to referencing the General theory of addiction Page 7

8 definition of a construct in another paper, with the important practical difference being that such links become computable. It is also possible that namespaces could be subject to arbitrary (in the scientific sense) bias. For example, a powerful research group might insist on use of a particular namespace for anyone wanting to receive research funding or publish their work in a prestigious journal. Again, this is no different from what already pertains with models and theories that are expressed in natural language. The key difference is that everything is made explicit so there is a greater opportunity to expose this tendency through automated information searches and to challenge it with analysis and evidence. The concept of a namespace allows for a diversity of perspectives on the Semantic Web, and ensures that anyone can say anything about anything. There are, however, practical limits to diversity. In order to have debates about the construct of addiction, for example, we do need to agree on the meaning of a set of more fundamental terms and predicates we use in that debate: terms such as is a subclass of and has attribute (see Table 2 above). The Semantic Web as a whole is underpinned by the existence of shared namespaces in which fundamental terms such as these are defined. The namespaces associated with RDF and OWL, the Web Ontology Language, provide a suite of general-purpose computable vocabulary elements that are needed to say anything at all. In the next section, we will explore the potential to use other preexisting ontologies in the development of a general theory of addiction. Building on existing ontologies Ontologies have been developed covering a range of topics in science, humanities and computing, and there is a very strong case for using existing, broader ontologies such as these wherever possible. Of most direct relevance to the study of addiction are the Mental Functioning Ontology (MF) (Hastings, Smith, Ceusters, Jensen, & Mulligan, 2012) and the WHO s International Classification of Functioning Disability and Health (Üstün, Chatterji, Bickenbach, Kostanjsek, & Schneider, 2003). Concepts in the field of addiction can be represented by linking constructs from relevant ontologies (Hastings, Le Novère, Ceusters, Mulligan, & Smith, 2012); and addiction ontologies can be developed using principles established by the Basic Formal Ontology (BFO) (Ceusters & Smith, 2010) and implemented by the Open Biological and Biomedical Ontologies (OBO) Foundry (Smith et al., 2007). We now take a closer look at the potential value of two existing ontologies in developing a general theory of addiction: the Basic Formal Ontology (BFO) and the Ontology for General Medical Science (OGMS). It should be noted that BFO represents only one of a number of top level ontologies. It is not static, and operates under the philosophy that if changes are required in order to be able to create better models, these can be implemented. Basic Formal Ontology BFO has been developed to provide a top level ontology for use in domain-specific ontologies. The purpose of BFO is to provide a categorisation of the sorts of things that exist that is generic enough to be agreed on across multiple domains, thereby providing a common vocabulary which ensures computers will be able to interpret relevant general distinctions in the same way across the different domain implementations. The Mental Functioning Ontology has been developed as one such domain-specific ontology and has been implemented in OWL. These make a suitable starting point for addiction ontologies. General theory of addiction Page 8

9 Figure 2: Main classes in the Basic Formal Ontology. Arrows represent the subclass of relationship. There is not the space to describe them in detail but it is worth outlining key features of BFO to illustrate how an addiction ontology may be constructed. Figure 2 shows the key terms in this ontology. The terms will look unfamiliar but they represent fundamental ways of representing the world using a language that makes sense once one understands the definitions. We provide a short outline of how the BFO understands various ontological categories. It is important to note the following descriptions are specific to the BFO and may not therefore correspond with the way in which ontological categories have been analyzed within other literatures, such as the literature on metaphysics within analytic philosophy. BFO starts with the concept of entity. An entity is anything that exists. Entities come in two forms: continuants and occurrents. Continuants exist over a period of time and come in the form of independent continuants such as objects, and dependent continuants such as dispositions and qualities. Occurrents inherently involve change over time, and include processes and process boundaries. BFO does not distinguish between processes and events, on the basis that events necessarily involve changes over time (e.g. onsets and offsets). The dichotomy between continuants and occurrents represents two different mutually interdependent perspectives on the world: physical and abstract objects or collections of objects and their features, and processes and events that transform these over time. Any theory or model of the physical world, including addiction, needs to be clear about where each of its constituent constructs lies in this world view. Thus craving can be construed as a relational quality that has an existence over time, is necessarily attached to a person who is an object and only exists in relation to something that is being craved. As such it can be measured and can be statistically and causally associated with other qualities such as stress and processes boundaries such as relapse. But the term craving can also refer to a process involving neural activity within, say, the nucleus accumbens. It can also refer to a disposition which is a latent characteristic that becomes expressed under certain conditions. All three of these definitions are meaningful and potentially useful, but failing to be clear about which of them one is referring to on a given occasion causes confusion. General theory of addiction Page 9

10 The other top-level dichotomy in BFO is between universals and instances (noting that this use of terms is different from that made in metaphysics). Universals are types or classes of entity that can exist or occur, while instances are specific cases. Thus addiction is a class of disposition whereas the addiction of John Smith to cocaine is an instance. This distinction between universals and specifics is fundamental in any model of the world, but needs to be recognised in every case where a model of addiction is used. Ambiguity in common language, and therefore in informal models and theories of addiction, can lead to confusion and unhelpful disagreement. Thus the universal statement Addiction to cocaine is caused by feelings of euphoria experienced after using the drug is ambiguous as whether this refers to all instances of addiction or some instances, or all instances of cocaine-induced euphoria or some cases. Most, if not all, of the subtypes of entities in BFO are potentially relevant to building models of addiction. The entity type history is particularly relevant as a subtype of process that is the sum of processes taking place in a continuant entity over time. Thus one may heuristically define the history of someone who for a period in his or her life suffers from addiction in terms of a period when he or she had not been engaged in the addictive behaviour (e.g. never used as substance), when he or she had engaged in the behaviour but did not show signs of addiction (e.g. recreational use of a substance), when he or she experienced addiction to the behaviour (e.g. diagnosed as suffering from substance use disorder), when he or she was voluntarily abstinent but subject to repeated cravings (e.g. attempting recovery from substance use disorder), and when he or she was abstinent and not repeatedly experiencing cravings (e.g. in recovery from substance use disorder). A common confusion in models of mental entities is between functions and functioning. It is apparent from Figure 2 that functions are classified as types of disposition; they are latent enduring attributes of continuants that result in particular processes in specific circumstances. They may change over time but it is not inherent in their nature for them to change. Functions are dispositions that have been designed or evolved. They are extremely important in any model of human behaviour or psychology. For example, motivation can be regarded as a function of animals that has evolved to get animals to behave in ways that promote survival and reproduction. Motivational functioning (in the form of processes) is a realisation of variants of this function in specific circumstances, but it needs to be treated differently in the model. Failure to do so leads to confusion in the specification of measures and lack of clarity in predictions. A major feature of BFO needs to be considered when applying it to developing models and theories of addiction. Models of complex systems such as animals need to be able to handle the fact that there will be many instances that do not conform to a standard version. Thus we can define an elephant as a pachyderm with a trunk and large ears, but if an elephant loses its trunk in an accident it is still an elephant. This is handled in BFO with by creating canonical models and allowing for deviations that can be represented as needed, without having to specify every possible way in which deviations may occur in advance. This can be used in models of addiction by creating canonical models involving the entities and their relationships that are typically observed in addiction and only extending the model as needed to represent deviations from this canonical representation. From the preceding paragraphs it should be apparent that using BFO as the basis for a model or theory of addiction allows this to be anchored in a set of concepts that have been defined, promotes clarity in the use of terms, and enables different models or theories to be made explicit and compared with each other. BFO does not impose any domain-specific propositions on to models or theories made using it, beyond its very broad world view. General theory of addiction Page 10

11 Ontology for General Medical Science Many, if not most, people who work in the field of addiction find it useful to think of it as a form of clinical abnormality. The fact that it is included in the DSM and ICD diagnostic criteria illustrates this. Others may take a different view, but if one does want to model addiction as a form of clinical abnormality, it is helpful to base that model on the Ontology for General Medical Science (OGMS) which is itself based on BFO. Basing a model of addiction on OGMS helps to improve the clarity of the model and can make explicit differences between one model and another. OGMS distinguishes between clinical abnormality, disorder and disease. This distinction turns out to be extremely important for physical health as well as for mental health problems. OGMS introduces the entity life plan of an organism. Life plan is a canonical history of the organism and includes gestation, maturation, and aging. It then defines a clinical abnormality in terms of three things being present simultaneously: 1) it is not part of the life plan of an organism of a relevant type, 2) it is causally related to an elevated risk of pain or other feelings of illness, dysfunction or death, such that 3) the elevated risk exceeds a threshold level. This definition of clinical abnormality should look quite familiar to someone working in the field of addiction. There is nothing unique to addiction about it involving a deviation from an expected set of characteristics, this being linked to risk of harm, and there needing to be a threshold to determine whether this qualifies or is sub-clinical. In the case of physical clinical abnormalities, one may set the threshold as the level at which the person affected or someone who cares about him or her feels a need to see a health professional. In the case of mental abnormalities, we need to allow for the possibility that neither the person concerned nor his or her friends or family sees a problem but there is sufficient risk of harm that others can make the determination that something needs to be done. OGMS then defines a disorder as an independent continuant which is a causally linked combination of components that is clinically abnormal. The disorder is the clinical abnormality but it is not the disease itself. In the case of influenza, the disorder is the presence in the body of viable cells containing the influenza virus. The disease is a disposition that is realised in a pathological process (acute inflammation) that produces abnormal bodily features that are recognised as signs (e.g. fever) and symptoms (e.g. headache). Thus a disease is construed as a disposition of an organism to undergo pathological processes because of one or more disorders. The concept of disease course is introduced as the aggregate of pathological processes in which a disease is realised. Note that a disease is a continuant while a disease course is an occurrent the two differ at the very top level of BFO. While this may seem an artificial distinction, few would disagree that it is useful to be able to refer to an instance of addiction as a single entity which exists in a given person over time. It is useful to be able to refer to John Smith s cocaine addiction as such, despite the varied course that the addiction may take, through initial development, subsequent abstinence, relapse and recovery. In this formulation, addiction can be regarded as a mental disorder or set of mental disorders that are realised through pathological processes as a set of signs and symptoms. In that model addiction is not the pathological processes or the signs and symptoms which may vary in both quality and intensity as a function of a range of environmental, physiological and psychological factors. Addiction can be viewed alternatively as the whole disease process. This is a matter of perspective. What is important are the propositions about the disorders, processes, signs and symptoms that is General theory of addiction Page 11

12 where, as long as the propositions are properly formulated, there can be fruitful discussion about how they relate to the evidence. BFO and OGMS are examples of ontologies that can be expressed in a language such as OWL and used to build domain-specific ontologies designed to model phenomena such as addiction. Some people working in the field of addiction may consider them too restrictive in terms of the structure they impose on thinking in the area. On the other hand, there are advantages to building on them in an addiction ontology precisely because of the clarity and structure that they bring to the process of model and theory building. The key point is that it is up to people working in the field what they want to do. If they can come up with an ontology that better captures the phenomena observed in addiction or is more parsimonious or coherent, this will represent an advance. But in attempting to do so, it is wasteful and confusing to ignore ontologies that already exist rather their limitations need to be described and the ways that the novel ontology improve on them need to be explicitly stated. Conclusion: towards a general theory of addiction We need to move beyond current approaches to modelling addiction to one that involves entities and relationships that are clearly and explicitly defined. Given the nature of addiction, it is neither realistic nor desirable to attempt to impose a set of definitions to make this happen. The Semantic Web, however, offers a pragmatic way to achieve clarity of constructs and develop consensus, while at the same time recognising that divergent views have utility. Models of addiction can be expressed in terms of subject-predicate-object triples using Uniform Resources Identifiers (URIs) for each component. These URIs may link to a common term such as craving but defined differently in different namespaces. In this way differences between use of terms will be explicit. A considerable amount of work has already been done in other areas of science to specify concepts and relationships that can form a framework for models of addiction. In particular, the Basic Formal Ontology (BFO) provides top level constructs relating to entities that have continuing existence such as objects and their characteristics (called continuants ), and those that involve change over time such as processes (called occurrents ). Other top level frameworks may be appropriate, but developing models of addiction within the BFO will make use of the work that has been done and allow these models to be linked to others in related fields of study. What might this look like in practice? One approach to creating a general theory of addiction would be to establish an ontology of addiction with the option for those contributing to the ontology to create their own sub-ontologies with specified namespaces. Thus one might start with a medically based ontology elaborating the Mental Functioning and Mental Disease ontologies within one namespace and build others with other namespaces, adopting the principles of the OBO Foundry to maintain discipline and efficient working within the enterprise. This ontology would define entities relevant to addiction and their relationships, relate these to entities in other fields of study in multiple disciplines, and link these with empirical findings and measures. This would be a major undertaking but could be facilitated by the development of a web portal in which users could browse, contribute to, and query the developing ontology. The resulting ontology could then form the basis for one or more general theories of addiction. Many of the core propositions within the models and theories would be embodied within the ontology but it is likely that the representation of the models and theories would need to extend to forms of representation that are not well captured with this kind of data structure, such as equations, probabilistic formulations and conditionals. General theory of addiction Page 12

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