David Shotton ^§, Chris Catton and Graham Klyne

Image Bioinformatics Research Group, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK

^§ Corresponding author: e-mail: david.shotton@zoo.ox.ac.uk

“An ontology is a formal, explicit specification of a shared conceptualisation. ”

Abstract

Although ontologies have been created to cover a large number of domains, many of them have characteristics that make them difficult to reuse in other contexts.  If ontologies are to be reused easily by others, they should be constructed as sets of small modules, each of which is a simple well-defined subsumption hierarchy that is not over-burdened by domain and range constraints.  This will make it easy for others to re-use the ontology without incurring unintended logical entailments.  In contrast, when using ontologies for particular applications such as the support of knowledge management systems, they may need to be extended and constrained.  This should be done in a manner that does not obscure the interpretation of core metadata items expressed using the extended ontology, when these are read by applications that have no knowledge of those extensions.

Current ontology development practice

Development of the semantic web (http://www.scientificamerican.com/article.cfm?id=the-semantic-web;  http://doi.ieeecomputersociety.org/10.1109/MIS.2006.62) involves the creation of vocabularies and ontologies designed to describe particular domains of knowledge.  It is desirable that such an ontology, ideally encapsulating a consensus view among domain experts, can be used as part of a practical information management application, and can also be easily shared with the wider community.  This paper examines the conflicting design requirements for these two functions, and summarizes a design approach.

Gruber (http://tomgruber.org/writing/onto-design.htm) stated:

‘An ontology should require the minimal ontological commitment sufficient to support the intended knowledge sharing activities … An ontology serves a different purpose than a knowledge base, and therefore a different notion of representational adequacy [McCarthy and Hayes, 1969] applies’.

However, in the current wave of interest in the semantic web and ontologies, this distinction has been blurred.  Some ontologies available on the web are too tightly specified to be easily shared, and are closer in spirit to knowledge bases than to Gruber’s idea of an ontology.

We believe this is the result of two forces:

a)      The current state of tools and standards for ontology development means that we are only just reached the point where different ontologies can be imported and made to work together easily and reliably.

b)      In developing the semantic web, much effort has been expended on producing stand-alone ontologies for use in specific systems, but less on designing them for general applicability so that they may be widely and easily shared.

If the semantic web is to become more than a collection of isolated knowledge-base systems, we need to be able to re-use and combine ontologies more easily.  There are several reasons for using ontologies created by others rather than writing them ourselves: it saves time and money, it permits each domain to be described accurately by appropriate domain specialists, and most importantly perhaps, it permits larger communities to come to a common agreement on the meaning of specific terms.

However, many ontologies are difficult or impossible to re-use, because they are too complex or over-committed to a particular application, contain inadequate or incomplete class annotations, are poorly structured, express a philosophy that is at odds with that of the importing ontology, or are over-restricted.

a)      Ontology too complex for easy re-use Many ontologies are very large.  For example, CYC (http://www.cyc.com/cyc/technology/whatiscyc) contains over a million assertions, the Gene Ontology (GO) has almost 30,000 terms, while the National Cancer Institute Thesaurus (http://www.mindswap.org/2003/CancerOntology/; http://ncimeta.nci.nih.gov) in 2003 contained about 26,000 concepts and about 71,000 terms (http://www.mindswap.org/papers/WebSemantics-NCI.pdf).  Large size is appropriate when the ontology is an application ontology used to describe a large domain (e.g. GO): such ontologies benefit from having simple structures.  Possibilities for re-use are limited if the ontology in question is too complex or unwieldy.  If the need is just to be able to re-use a few classes, importing a large existing ontology bodily into a new ontology is problematic, since it obscures structure and takes up resources.

Our experience has been that existing published ontologies may also suffer from trivial faults:

• they may contain a rich mixture of dissimilar class types that are not clearly differentiated;
• they may contain class duplications and logical inconsistencies that have gone unrecognized because of its large size; and
• they may contain classes that properly belong in separate third-party ontologies, not in the domain-specific ontology under consideration.

In contrast, a number of vocabularies, including Dublin Core (http://dublincore.org/documents/dcmi-terms/), FOAF (http://xmlns.com/foaf/spec/) and SiOC (http://sioc-project.org/ontology), which might be described a generic rather than domain-specific, have found widespread use by virtue of their simplicity. The distinction between application ontologies and reference ontologies is clarified in a related article, Reference and Application Ontologies, by James Malone and Helen Parkinson.

b)      Ontology annotations incomplete or inadequate Ontology class annotations provide an important link between human- and machine-processable knowledge, and for this reason are important components of any ontology.  In some ontologies, the vaguensss or inappropriate implied meaning of some of the class names used, coupled with inadequate annotation, rendered them confusing.  Other ontologies, while having class names that might seem appropriate for re-use, fail to provide annotations for their classes and properties, or use annotations that are inadequate.  For example, the ka (Knowledge Acquisition) ontology (http://www.cs.man.ac.uk/~horrocks/OWL/Ontologies/ka.owl) contains no human-readable descriptions of the classes and properties, making it difficult to use without lots of cross-referencing to other documents, while in BIBO, the bibliographic ontology (http://bibliontology.com/), the class Standard is inadequately annotated as “A document describing a standard”.  Many other annotations are taken directly from Wikipedia, and are not always appropriate.  CIDOC CRM (http://cidoc.ics.forth.gr/) is an example of an ontology with generally good annotations.

c)      Ontology poorly structured For example, in the Bibtex ontology  http://oaei.ontologymatching.org/2004/Contest/301/onto.html) the hasAuthor property is a datatype property with a range of String, which effectively prevents one from identifying an author as a member of the class Person.  Furthermore, it cannot be used to describe an ordered list of authors in an easily machine-processable fashion.

d)      Ontology expresses an alternative philosophy Occasionally, different ontologies express alternative views about a domain, making it difficult to use elements of one easily within the other.  For example, while CiTO, the Citation Typing Ontology (http://purl.org/net/cito/) adopts the Works, Expressions, Manifestations hierarchy of the FRBR (Functional Requirements for Bibliographic Records; http://www.ifla.org/VII/s13/frbr/frbr1.htm) classification model developed by the United States Library of Congress to characterize different aspects of a publication, BIBO does not.  While CiTO has Work: cito:ResearchPaper; Expression: cito:JournalArticle, BIBO has bibo:AcademicArticle, which conflated these two concepts.  This makes it difficult to re-use BIBO classes within CiTO.

e)      Ontology too committed Perhaps the main problem is that many ontologies commit secondary users to modelling patterns that may be inappropriate to their needs. For example there is no consensus between bibliographic ontologies as to whether a document is authored by a set of people, an ordered list of people, or an ‘agent’ that may be a corporate body, situations brought about by varying restrictions on the range of the property hasAuthor.  This point relates to the previous one, since over-commitment becomes a problem in the face of alternative philosophies.

Thus, although different ontological representations now abound, many pose problems when it comes to re-using them in other ontologies.  The alternative approach of defining equivalences between terms in different ontologies suffers from some of the same problems, since use of owl:EquivalentClass is logically strict.  Strict equivalence is inappropriate if the definitions of the classes within the two ontologies differ significantly.  For example, in FRBR a Work is a distinct intellectual or artistic creation, an abstract concept recognised through its various expressions.  However, in CiTO the definition of cito:Work is restricted to works that cite or may be cited, primarily works of scholarship that contain bibliographic references, rather than artistic works such as plays or photographs that do not.  Thus cito:Work is classified as a subclass of frbr:Work, not an equivalent class.  Such subclassing should be undertaken with caution, since it implies logical entailment of any domain and range restrictions of the superclass.  An alternative is just to indicate that some sort of relationship exists between classes in two ontologies, for example by use of rdfs:seeAlso (http://www.w3.org/TR/rdf-schema/).

Ontologies for sharing, ontologies for use

To avoid these potential problems, we propose the following principles of ontology design to maximize the reusability of ontologies by third parties.  Modifying existing ontologies so that their public shared versions conform to these requirements is best undertaken collaboratively by a group of domain experts.

1. Ontologies designed for sharing within a community should be kept small, or have a simple structure that uses just a few properties and is not deeply nested. It is easier to achieve consensus about a small ontology on a single topic that is small enough to be comprehended in its entirety, just because there are fewer assertions to disagree about.  Small and/or simple ontologies are easier to incorporate by people wishing to use them for third-party applications.
2. Classes designed for sharing should form simple subsumption (is_a) hierarchies in which sibling classes are disjoint from each other, and where each class represents a single notion.  This makes the ontologies easier to manage and easier to validate by domain experts.
3. It follows from this that in any collection of related small ontology modules, covering a particular domain of knowledge, each class must be defined in only one ontology module.  If a class appears in two modules, it probably conflates two notions which need to be distinguished and separated.
4. All classes and properties should be annotated with clear and informative human-readable definitions, containing links to real-world examples where appropriate.
5. Ontology building is an empirical activity – while each ontology must be fit for purpose, clear and unambiguous, it need not capture the totality of applicable knowledge.
6. When writing an ontology that covers a small, specialist area, one should not assert anything that is not essential to the intended meaning.  In particular, one should think carefully before specifying the domain and range of a property, since this may cause problems for others extending from the ontology.
7. Third-party ontologies should be used wherever they are available and suitable, since there is no point in re-inventing wheels.
8. Ontologies should be written in a standard ontology language such as OWL, and validated of the ontology modules using an appropriate reasoner, for example FaCT++ (http://owl.man.ac.uk/factplusplus/).
9. More complex ontologies for specific applications (“knowledge bases” in the sense used by Gruber) can be built by combining smaller ontologies, and then by adding restrictions to enrich their meaning.

This statement of principles reflects the insights on ontology normalization first made by the Manchester ontologist Professor Alan Rector (http://www.cs.man.ac.uk/~rector/papers/rector-modularisation-kcap-2003-distrib.pdf).

To summarize, we propose the following distinctions:

A public shared ontology, as far as possible:

• should act primarily as a structured defined vocabulary;
• should define a limited domain;
• should be a simple subsumption hierarchy with disjoint sibling classes;
• should be sparing in its use of other ontologies;
• should be written in a standard ontology language such as OWL; and
• should have detailed human-readable annotations of the intended meaning of each term.

An application-level ontology, as far as possible:

• should be based upon or import one or more public ontologies describing particular domains;
• should restrict and/or cross-relate the public ontologies, thereby enabling more powerful reasoning to be used within a particular application;
• should extend the public ontology with new classes carefully, enabling more specific descriptions to be made without compromising the ability of third party applications that are not aware of these extensions to make sense of metadata thus encoded; and
• should ideally also be expressed in OWL-DL, to permit use of a Description Logic reasoner both for validation and for inference of additional relationships defined by the restrictions.

Once ontologies have been made suitable for public sharing and re-use, they should be published on an open access web server, or in an appropriate ontology warehouse such as the Open Biomedical Ontologies or SchemaWeb.  Application-level ontologies are still ‘shared conceptualizations’, but are now shared implicitly, by the users of the application.

Combining modular ontologies

One of the main advantages of producing modular ontologies is that not only can they be extended and constrained for a new purpose, but that they can also be combined easily to produce new or more comprehensive ontologies.  This process is made considerably easier when the modules are built with the same design principles and related to a common upper level ontology. The benefits of common design principles are emphasised by Open Biomedical Ontologies (http://www.obofoundry.org/).  Upper level ontologies are described in the accompanying paper, Upper Level Ontologies, by Frank Gibson.

Pitfalls remain, however, when attempting to combining existing ontologies.  To take a trivial example, semantic integration within the bibliographic community could be achieved with respect to the identification of bibliographic entities themselves by common usage of the Dublin Core class dc:title, which is well defined.  However, FOAF uses the class foaf:title to denote the prefix to a person’s name used to signify veneration, an official position, or a professional or academic qualification (Reverend, Dame, President, Dr, Professor, etc.).  While dc:title and foaf:title are logically distinct, we need to be careful to avoid human misinterpretation of ‘title’, if both are used in the same ontology.

At a deeper level, we need to avoid assumptions that lead to semantic misalignment of terms.  For example, it would be incorrect in many cultures to equate “family name” with “last name”.  A biological example is given by our work with Drosophila genomics data.  The FlyTED Database contains in situ hybridization images of gene expression in Drosophila testis.  In it, each tagged sequence probe used to identify the location of messenger RNA in the specimens, was, for convenience, originally described using the name of gene from which the mRNA was transcribed.  However, because of subsequent gene re-assignments within FlyBase, the genomic database for Drosophila, mismatches developed between the “genes” in FlyTED and the corresponding gene identifiers in FlyBase.  This caused subsequent inconsistencies when combining results from the two databases as part of our OpenFlyData Services, which were resolved by recognising the “gene” names in FlyTED as being distinct from the FlyBase IDs, rather than synonymous, and then by introducing a mapping between them.

There are other problems to be resolved when one has to choose between a number of overlapping ontologies that could be used within an integration, but between which there are significant differences.  How is one to choose between them, or reconcile their differences?  These are issues for which there is no clear prescriptive answers, and for which best practice is being worked out within the ontology community.  Related issues of semantic data integration are described in the article Semantic Integration in the Life Sciences by Allyson Lister.

Extending shared ontologies

If extension of a ‘public’ ontology is required for a particular application, it is advisable to extend it only by adding new sub-classes at the edges,  rather than by modifying the meaning of more central ontological entities.  If this is done, third party applications, able to understand the public ontology but having no knowledge of these extensions, will still be able to understand correctly the core metadata created using the extended ontology.  We exemplify this from our recent work with the CIDOC Conceptual Reference Model (CIDOC CRM; http://cidoc.ics.forth.gr/), an ontology developed for the museum community specifically to describe cultural heritage information.

In the CLAROS Project (http://www.clarosnet.org/; http://imageweb.zoo.ox.ac.uk/pub/2009/publications/Kurtz_Parker_Shotton_et_al-IEEE_CLAROS_paper.pdf), we have used the CIDOC CRM to provide a common framework onto which to map the data models of a number of academic resources describing classical art objects, and have then created a data web integrating information from these resources into a single user interface.  For this, we have employed the RDF version of CIDOC CRM from Erlangen University (http://purl.org/NET/crm-owl).

Dating in antiquity is not an exact science, and we needed to capture this uncertainty in our CLAROS metadata.  For this, we extended CIDOC CRM to permit us to record the estimated bounds of the inaccuracy relating to dates of creation of classical art objects.  We could have done this by introducing new properties to relate existing CIDOC CRM classes.  However, the problem with this approach is that baseline applications (without appropriate inference support) won’t know about these new properties, so the nature of the relationships that they might understand is lost, and sections of the knowledge graph might become disconnected for such applications. The alternative approach we chose to adopt was just to add new nodes at the edge of the CRM graph, and then add new properties relating these to instances of existing classes.  This left the original classes connected only by original properties. Furthermore, we supplemented our new date range metadata with more general statements that do not depend on knowledge of these extensions, accessible to ‘standard’ CIDOC CRM applications.

Specifically, we created the classes claros:not_before and claros:not_after, that have the CRM class E61.Time_Primitive class as their domain, and used these, together with a ‘label’ time range statement for display use by applications that understand only the non-extended CRM terms, thus (in RDF N3 notation):

. . .

[ rdf:type crm:E61.Time_Primative class ;

claros:not_before “-525″۸۸xsd:gyear ;

claros:not_after “-475″۸۸xsd:gyear ;

rdfs:label “about 500 BC” ] .

The need for better tools

The ontology writing and editing tools Protégé version 4 (http://protege.stanford.edu/) and OBOedit version 2 (http://oboedit.org/) are powerful and sophisticated – see articles on Protégé & Protégé-OWL by Alan Rector and on OBO Format by David Osumi-Sutherland.  However, there is still a need for good tools in four areas to assist those creating, integrating and using ontologies:

• Tools to assist in the early stages of ontology development, using text mining and natural language processing to extract relevant terms from the scientific literature that can then be organized into ontology modules.
• Tools that allow vocabulary designers to capture, refine and ultimately formalize their intuitions without being forced to deal with distracting logical details early in the design process.
• Tools to permit the comparison of different ontologies, and of different versions of a single ontology under development, and to visualize these differences in a readily comprehensible manner.
• Tools to assist in the location of relevant third-party ontologies for integration and use.  The National Centre for Biomedical Ontology’s BioPortal, and the European Bioinformatics Institute’s Ontology Lookup Service works well for the biological domain, but there are no equivalents for more generic ontologies such as the Information Artifact Ontology, the Citation Typing Ontology, FRBR, FOAF and CIDOC CRM.  One just has to learn about these by reading, attending meetings, word of mouth and experience.  This makes entry into this field extremely difficult for newcomers.

Conclusion

Applying the principles outlined here should make it easier to create an ontology de novo, or to take an existing ‘tangled’ ontology, normalize it, and ‘bring it to market’.  Such public ontologies, being simpler and more loosely defined, should gain a far wider degree of consensus and obtain greater usage.  The mechanics of this ontology normalization process are described in a separate article entitled Automatic Maintenance of Multiple Inheritance Ontologies by Mikel Egaña Aranguren, while the construction of application ontologies is described in the related articles Community Driven Ontology Development by James Malone.

Acknowledgements

This paper is a restatement and expansion of ideas first employed during the Second Animal Behavior Metadata Workshop held at Cornell University in September 2005, posted here.

This paper is an open access work distributed under the terms of the Creative Commons Attribution License 3.0 (http://creativecommons.org/licenses/by/3.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided that the original author and source are attributed.

The paper and its publication environment form part of the work of the Ontogenesis Network, supported by EPSRC grant EP/E021352/1.

Robert Stevens*, Alan Rector* and Duncan Hull†

* School of Computer Science, The University of Manchester, Oxford Road, Manchester, UK
† EMBL Outstation – Hinxton, European Bioinformatics Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge, CB10 1SD, UK

Defining ontology

In OntoGenesis we intend to provide biologists and bioinformaticians with the means to understand ontologies within the context of biological data; their nature; use; how they are built; and some of those bio-ontologies that exist. All of this is based upon knowing what an ontology is. This can then lead on to the motivation for their use in biology; how they are used; and so on. The definition of ontology is disputed and this is confounded by computer scientists having re-used and re-defined a discipline of philosophy. The definition here will not suit a lot of people and upset many (especially use of the word “concept”); We make no apology for this situation, only noting that the argument can take up resources better used in helping biologists describe and use their data more effectively.

In informatics and computer science, an ontology is a representation of the shared background knowledge for a community. Very broadly, it is a model of the common entities that need to be understood in order for some group of software systems and their users to function and communicate at the level required for a set of tasks. In doing so, an ontology provides the intended meaning of a formal vocabulary used to describe a certain conceptualisation of objects in a domain of interest. An ontology describes the categories of objects described in a body of data and the relationships between those objects and the relationships between those categories. In doing so, an ontology describes those objects and sometimes defines what is needed to be known in order to recognise one of those objects. An ontology should be distinguished from thesauri, classification schemes and other simple knowledge organisation systems. By controlling the labels given to the categories in an ontology, a controlled vocabulary can be delivered; though an ontology is not a controlled vocabulary. when represented as a set of logical axioms with a strict semantics, an ontology can be used to make inferences about the objects that it describes and consequently provides a means to symbolically manipulate knowledge.

In philosophy, ontology is a term with its origins with Aristotle in his writings on Metaphysics, IV,1 from 437 BCE. In very general terms, it is a branch of philosophy concerned with that which exists; that is, a description of the things in the world. Philosophers in this field tend to be concerned with understanding what it means to be a particular thing in the world; that is, the nature of the entity. The goal is to achieve a complete and true account of reality. Computer scientists have taken the term and somewhat re-defined it, removing the more philosophical aspects and concentrating upon the notion of a shared understanding or specification of the concepts of interest in a domain of information that can be used by both computer and humans to describe and process that information. The goal with a computer science ontology is to make knowledge of a domain computationally useful. There is less concern with a true account of reality as it is information that is being processed, not reality. The definition used here (and any other definition for that matter) is contentious and many will disagree with it. Within the bio-ontology community there are those that take a much more philosophical stance on ontology. The OBO Foundary, for instance, do take a much more philosophical view.

Putting the string “define:ontology” into the Google search engine finds some twenty or so definitions of ontology. They all cluster around either a philosophical or a computer science definition of ontology. This is presumably the root of the jibe that ontology is all about definitions, but there is no definition of ontology. So, we should really distinguish between philosophical ontology and computer science ontology and remove some of the dispute. Tom Gruber has one of the most widely cited definitions of ontology in computer science, though conceptual models of various types have been built within computer science for decades. Tom Gruber’s definition is:

“In the context of knowledge sharing, the term ontology means a specification of a conceptualisation. That is, an ontology is a description (like a formal specification of a program) of the concepts and relationships that can exist for an agent or a community of agents. This definition is consistent with the usage of ontology as set-of-concept-definitions, but more general. And it is certainly a different sense of the word than its use in philosophy.” DOI:10.1006/knac.1993.1008 DOI:10.1006/ijhc.1995.1081

The most noteworthy point is that Gruber states that his definition of ontology is not “ontology in the philosophical sense”. Nevertheless, computer science ontology is still informed by the philosophical, but the goals for their creation and use are different.

An important part of any ontology is the individuals or objects. There are trees, flowers, the sky, stones, animals, etc. As well as these material objects, there are also immaterial objects, such as ideas, spaces, representations of real things, etc. In the world of molecular biology and beyond, we wish to understand the nature, distinctions between and interactions of objects such as: Small and macromolecules; their functionalities; the cells in which they are made and work; together with the pieces of those cells; the tissues these cells aggregate to form; etc, etc. We do this through data collected about these phenomena and consequently we wish to describe the objects described in those data.

As human beings, we put these objects into categories or classes. These categories are a description of that which is described in a body of data. The categories themselves are a human conception. We live in a world of objects, but the categories into which humans put them are merely a way of describing the world; they do not themselves exist; they are a conceptualisation. The categories in an ontology are a representation of these concepts. The drive to categorise is not restricted to scientists; all human beings seem to indulge in the activity. If a community agrees upon which categories of objects exist in the world, then a shared understanding has been created.

In order to communicate about these categories, as we have already seen, we need to give them labels. A collection of labels for the categories of interest forms a vocabulary or lexicon. Human beings can give multiple labels to each of these categories. This habit of giving multiple labels to the same category and the converse of giving the same label to different categories polysemy) leads to grave problems when trying to use the descriptions of objects in biological data resources. This issue is one of the most powerful motivations for the use of ontologies within bioinformatics.

As well as agreeing on the categories in which we will place the objects of interest described in our data, we can also agree upon what the labels are for these categories. This has obvious advantages for communications – knowing to which category of objects a particular label has been given. This is an essential part of the shared understanding. By agreeing upon these labels and committing to their use, a community creates a controlled vocabulary.

The objects of these categories can be related to each other. When each and every member of one category or class is also the member of another category or class, then the former is subsumed by the latter or forms a subclass of the superclass. This subclass superclass relationship between objects is variously known as the “is-a” DOI:10.1109/MC.1983.1654194, subsumption or taxonomic relationship. There can be more than one subclass for any given class. If every single kind of subclass is known, then the description is exhaustive or covered. Also, any pair of subclasses may overlap in their extent, that is, share some objects, or they may be mutually exclusive, in which case they are said to be disjoint. Both philosophical and ontology engineering best practice often advocate keeping sibling classes pairwise disjoint.

As well as the is-a relationship, objects can be related to each other by many other kinds of relationship DOI:10.1186/gb-2005-6-5-r46. One of the most frequently used is the partOf relationship, which is used to describe how objects are parts of, components of, regions of, etc. of other objects. Other relationships will describe how one object developsInTo or is transformed into another object, whilst retaining its identity (such as tadpole to frog). The deriveFrom relationship describes how one object changes into another object with a change of identity. Another relationship describes how a discrete object can ParticipateIn a process object.

These relationships, particularly the is-a relationship give structure to a description of a world of objects. The relationships, like the categories whose instances they relate, also have labels. Relationship labels are another part of a vocabulary. The structured description of objects also gives a structured controlled vocabulary.

So far, we have only described relationships that make some statement about the objects being described. It is also possible to make statements about the categories or classes. When describing the elemental form of an atom, for example, `Helium’, statements about the discovery date, industrial uses, are about the category or class, not about the objects in the class. Each instance of a `Helium’ object was not discovered in 1903; most helium atoms existed prior to that date, but humans discovered and labelled that category at that date.

Ideally, we wish to know how to recognise members of these categories. That is, we define what it is to be a member of a category. When describing the relationships held by an object in a category, we put inclusion conditions upon those instances or category membership criteria. We divide these conditions into two sorts:

1. Necessary Conditions: These are conditions that an object must fulfil, but fulfilling that condition is not enough to recognise an object as being a member of a particular category.
2. Necessary and Sufficient Conditions: These are conditions that an object must fulfil and are also sufficient to recognise an object to be a member of a particular category.

For example, in an ontology of small molecules such as Chemical Entities of Biological Interest (ChEBI) DOI:10.1093/nar/gkm791 has a definition of alcohol and there are several ways of defining what this means. Each and every organic molecule of alcohol must have a hydroxyl group. That an organic molecule has a hydroxyl substituent is not, however, enough to make that molecule an alcohol. If, however, an organic molecule has a saturated backbone and a hydroxyl substituent on that backbone is enough to recognise an alcohol (at least according to the IUPAC “Gold Book”).

In making such definitions, an ontology makes distinctions. A formal ontology makes these distinctions rigourously. Broad ontological distinctions would include those between Continuant and Occurant; that is, between entities (things we can put in our hands) and processes. Continuants take part in processes and processes have participants that are continuants. Another distinction would be between Dependent and Independent objects. The existence of some objects depend on the existence of another object to bear that object. for example, a car is independent of the blue colour it bears. Continuants, for example, can be sub-categorised into material and immaterial continuants such as the skull and the cavity in the skull. Making such ontological distinctions primarily helps in choosing the relationships between the objects being described, as well as some level of consistency.

Capturing such descriptions, including the definitions forms an ontology. Representing these descriptions as a set of logical axioms with a strict semantics enables those descriptions to be reliably interpreted by both humans and computers. Forming a consensus on which categories should be used to describe a domain and agreeing on the definitions by which objects in those categories are recognised enables that knowledge to be shared.

The life sciences, unlike physics, has not yet reduced its laws and principles to mathematical formulae. It is not yet possible, as it is with physical observations, to take a biological observation, apply some equations and determine the nature of that observation and make predictions etc. Biologists record many facts about entities and from those facts make inferences. These facts are the knowledge about the domain of biology. This knowledge is held in the many databases and literature resources used in biology.

Due to human nature, the autonomous way in which these resources develop, the time span in which they develop, etc., the categories into which biologists put their objects and the labels used to describe those categories are highly heterogeneous. This heterogeneity makes the knowledge component of biological resources very difficult to use. Deep knowledge is required by human users and the scale and complexity of these data makes that task difficult. In addition, the computational use of this knowledge component is even more difficult, exacerbated by the overwhelmingly natural language representation of these knowledge facts.

In molecular biology, we are used to having nucleic acid and protein sequence data that are computationally amenable. There are good tools that inform a biologist when two sequences are similar. Any evolutionary inference based on that similarity, however, based upon knowledge about the characterised sequence. Use of this knowledge has been dependent on humans and reconciliation of all the differing labels and conceptualisations used in representing that knowledge is necessary. For example, in post-genomic biology, it is possible to compare the sequences of the genome and the proteins it encodes, but not to compare the functionality of those gene products.

There is, therefore, a need to have a common understanding of the categories of objects described in life sciences data and the labels used for those categories. In response to this need biologists have begun to create ontologies that describe the biological world. The initial move came from computer scientists who used ontologies to create knowledge bases that described the domain with high-fidelity; an example is EcoCyc http://view.ncbi.nlm.nih.gov/pubmed/8594595. Ontologies were also used in projects such as TAMBIS DOI:10.1147/sj.402.0532 to describe molecular biology and bioinformatics to reconcile diverse information sources and allow creation of rich queries over those resources. The explosion in activity came, however, in the post-genomic era with the advent of the Gene Ontology (GO) doi:10.1038/75556. The GO describes the major functional attributes of gene products – molecular function, biological process and cellular components. Now some forty plus genomic resources use GO to describe these aspects of the gene products of their respective organisms. Similarly, the Sequence Ontology describes sequence features; PATO (the phenotype Attribute and trait ontology) describes the qualities necessary to describe an organism’s phenotype. All these and more are part of the Open Biomedical Ontologies project (OBO) 10.1038/nbt1346.

Conclusion

In conclusion, we can say that there is a need to describe the entities existing within data generated by biologists so that they know what they are dealing with. This entails being able to define the categories of biological entities represented within those data. As well as describing the biological entities, we also need to describe the science by which they have been produced. This has become a large effort within the bioinformatics community. It has also been found to be a difficult task and much effort can be used in attempting to find the true nature of entities in biology and science. It should be remembered, however, that the goal of the bio-ontology effort is to allow biologists to use and analyse their data; building an ontology is not a goal in itself.

References

This text is adapted and updated from Ontologies in Biology by Robert Stevens. A numbered list of references will be generated from the DOI’s above in later drafts of this article after peer review.

Acknowledgements

This paper is an open access work distributed under the terms of the Creative Commons Attribution License 3.0, which permits unrestricted use, distribution, and reproduction in any medium, provided that the original author and source are attributed.

The paper and its publication environment form part of the work of the Ontogenesis Network, supported by EPSRC grant EP/E021352/1.