To paraphrase Davis, Shrobe & Szolovits , an ontological commitment is an answer to the question of “how should I think about the world?”. An ontology is a model or knowledge representation of a field of interest . By using an ontology, you are committing to its world view; if you borrow from an ontology, you should also be borrowing its commitment. That is, an ontology says “there are these objects and this is how they are related”; the ontology has made that commitment and, by dint of using that ontology, so are you. This has consequences for how ontologies are used and re-used. This kblog gives an introduction to ontological commitment and illustrates how it is worth bearing in mind as you use and re-use ontologies.
Bio-health Informatics Group
School of Computer Science
University of Manchester
School of Computing Science
Newcastle University+ Newcastle
wikipedia’s summary of ontological commitment  goes like this:
[… an] ontology refers to a specific vocabulary and a set of explicit assumptions about the meaning and usage of these words, then an ontological commitment is an agreement to use the shared vocabulary in a coherent and consistent manner within a specific context.
That is, when using an ontology there should be a commitment to its view on the field of interest it describes. This is the sharing or shared view that is often refered to in an information systems version of an ontology. To keep it succinct, when using terms (classes) and axioms from an ontology, one is buying into the world view of that ontology. An ontology is a theory of a field of interest ; that theory uses objects in its explanation of that field – those objects are its ontological commitment.
This is not only true when using an ontology for annotation, but when an ontology is authored – either de novo or when re-using another ontology.
The way knowledge is modelled makes a commitment to a world view. When modelling de novo the modelling style makes a statement about how the entities in the field of interest are arranged. When an existing domain neutral or top ontology  is used, a modeller is making a commitment to that upper ontology’s understanding of how entities are arranged. By, for example, using BFO, an ontologist is commiting to a view where continuants can have qualities, but processes may not; if DOLCE is used as a domain neutral ontology, a commitment is made, for example, to dividing qualities into qualia and qualities. It has to be added, that the user need not believe it in the slightest, but the bottom line is that in re-using another ontology or ontology fragment, that ontology’s interpretation of a field of interest should also be re-used.
These views provided by another’s ontology are not a la carte; they are more like the menu – you take the whole view or you leave it altogether. One shouldn’t pick and choose pieces of viewpoint from a variety of ontological commitments, especially radically different upper level, domain neutral, ontologies. One can’t just take the partial viewpoint (though one can take part of an ontology and not break ontological commitment) of an ontology to which one agrees; it’s the whole commitment or nothing – just as one cannot be a bit pregnant one cannot be a bit ontology x.
Another way of achieving an ontological commitment is by re-using parts of another ontology. For instance, by re-using relationships from the Relationships Ontology  an commitment is made to the ontological view taken on by that ontology fragment. When using those relationships, for instance, a modeler should commit to re-using them in the way intended. For example,
part_of should be used for describing the relationships between an entity and its parts, not for describing the relationship between an entity and that in which it is contained. Similarly, taking a class from another’s ontology and changing its definition and assuming it’s the same class doesn’t work; one commits to the donor ontology’s definition or one needs a new class.
By using an ontology I’m committing to its “world view”. When using, for instance, the Gene Ontology  to describe the major attributes of a gene product, the annotator should be committing to adopting the GO’s view of the world – making arbitrary decisions like “GO says x means blah,b but I’ll interpret x as meaning yadda yadda” leads to confusion and the world will collapse into a maelstrom of sin and corruption. Possible sins include:
In re-using a concept, one should look not only at the label of that concept, but its definition and its context to be sure the ontological commitments being made are the ones you want.
When we say that in using an ontology, you should effectively commit to it, this does not, of course, mean that you should use all of the ontology, in the sense of referring to all the terms in that ontology. In fact, many ontologies, refer to very few of the terms in the domain neutral ontologies that they import.
There are also important practical considerations; reasons why you might not wish to commit to all of an ontology. Consider, for example, an OWL ontology . If we import another ontology into ours, we get all of the axioms from the imported ontology whether we like it or not. This can be extremely problematic if, for instance, the other ontology is very large and we wish to use a single term. An obvious use case, is refering to
Homo sapiens from the NCBI taxonomy. There are a number of ways to address this problem; tools associated with MIREOT for example, will extract just the terms you need. This is a relatively safe thing to do, but there are still some potential pitfalls. For example, if you just import
ViridiPlantae (green plants) and
Homo sapiens, and then make one a subclass of the other, your ontology will be consistant. But if anyone else imports both your ontology and the NCBI taxonomy, these statements will be see to contradict. In this case you’re also breaking the ontological commitment to the NCBI taxonomy, where plants and humans do not have this relationship.
There are also reasons why you might wish to add new axioms describing terms from an ontology that you import. For example, say you import the Pizza Ontology , and discover the lack of labels in Italian is problematic; you could add these annotations in a second ontology. Another common reason for adding more axioms to an existing ontology is to switch OWL profile. For example, versions of DOLCE can be found in the EL, QL and DL, all refering to the same terms. The DL version contains axioms which will disallow statements that would be valid against the EL version.
In each of these cases, the practical considerations must be weighed up against the potential consequences. Ignoring axioms will mean that you are also ignoring potential inconsistencies; adding multi-lingual labels may be safe, but equally make break software which is expecting only English; adding axioms to terms from another ontology may cause undesired consequences in a third.
In general, the rule should be that if you ignore part of, or add to another ontology you should be careful to retain as much ontological commitment as you can by not contradicting the original.
Davis, Shrobe & Szolovits  talk of layers of commitment; what’s gone above is towards a “top” layer. Down at the bottom layer, we have the ontological commitment of the representation language itself. An knowledge representation language makes it possible to say some things about a field of interest and not others. Thus an ontology’s language is itself making an ontological commitment; it takes a view upon what things are important in the world and it is these that can be represented.
Moving up a layer, the modelling choices made in the ontology are making a commitment about the field of interest; by classifying aalong one primary axis as opposed to another, an ontologist is saying what he or she believes to be important about that aspect of the domain. By using, re-using or extending that ontology a modeller should “buy-in” to that commitment and is implicitly doing so by taking on the task.
An ontology makes commitments about the world it is representing. When using an ontology one is taking on that ontological commitment – there should be no “ontology x makes interpretation y, but I’ll make interpretation z”. Similarly, an ontology’s commitments are not a la carte, one cannot just leave out the bits of a point of view one doesn’t like and re-use the bits of a viewpoint one does like – by committing to a bit one is committing to the whole (though one, of course, doesn’t have to commit to the whole thing in a mechanical sense – that is, import the whole thing). In summary, an ontology is about shared, common understanding and ontological commitment is one of its central aspects. As ontologies are used and re-used, especially when re-using fragments of an ontology, ontological commitment should be born in mind.