An Overview of OntoClean
OntoClean is a methodology for validating the ontological adequacy and logical consistency of taxonomic relationships. It is based on highly general ontological notions drawn from philosophy, like essence, identity, and unity, which are used to elicit and characterize the intended meaning of properties, classes, and relations making up an ontology. These aspects are represented by formal metaproperties, which impose several constraints on the taxonomic relationships between concepts. The analysis of these constraints helps in evaluating and validating the choices made. In this chapter we present an informal overview of the philosophical notions involved and their role in OntoClean, review some common ontological pitfalls, and walk through the example that has appeared in pieces in previous papers and has been the basis of numerous tutorials and talks.
The OntoClean methodology was first introduced in a series of conference-length papers in 2000 [4, 7, 6, 12], and received much attention and use in subsequent years. The main contribution of OntoClean was the beginning of a formal foundation for ontological analysis. Alan Rector, a seasoned veteran at ontological analysis in the medical domain, said of OntoClean, “…what you have done is reduce the amount of time I spend arguing with doctors that the way I want to model the world is right…” . A similar comment came from the CYC people attending our AAAI-2000 tutorial: “You showed why the heuristic choices we adopted were right.” Most experienced domain modelers can see the correct way to, e.g., structure a taxonomy, but are typically unable to justify themselves to others. OntoClean has provided a logical basis for arguing against the most common modeling pitfalls, and arguing for what we have called “clean ontologies.”
In this chapter we present an informal overview of the four basic notions essence, identity, unity, anddependence, and their role in OntoClean, review the basic ontology pitfalls, and walk through the example that has appeared in pieces in previous papers and has been the basis of numerous tutorials and talks beginning with AAAI-2000.
The basic notions in OntoClean were not new, but existed in philosophy for some time. Indeed, the practice of modeling the world for information systems has many parallels in philosophy, whose scholars have been trying to describe the universe in a formal, logical way since the time of Aristotle. Philosophers have struggled with deep problems of existence, such as God, life and death, or whether a statue and the marble from which it is made are the same** entity. While these problems may seem irrelevant to the designer of an information system, we found that the conceptual analysis and the techniques used to attack these problems are not, and form the basis of our methodology.
1.2 Properties, Classes, and Subsumption
Many terms have been borrowed by computer science from mathematics and logic, but unfortunately this borrowing has resulted often in a skewed meaning. In particular, the terms property andclass are used in computer science with often drastically different meanings from the original. The use of the term property in RDF is an example of such unfortunate deviation from the usual logical sense.
In this chapter, we shall consider properties as the meanings (or intensions) of expressions like being an apple or being a table, which correspond to unary predicates in first-order logic. Given a particular state of affairs (or possible world, if you prefer), we can associate to each property a class (its extension), which is the set of entities that exhibit that property in that particular situation. The members of this class will be called instances of the property. Classes are therefore sets of entities that share a property in common; they are the extensional counterpart of properties. In the following, we shall refer most of the time to properties rather than classes or predicates, to stress the fact that their ontological nature (characterized by means of meta-properties) does not depend on syntactic choices (as it would be for predicates), nor on specific states of affairs (as it would be for classes).
The independence of properties from states of affairs gives us the opportunity to make clear the meaning of the term subsumption we shall adopt in this paper. A property p subsumes q if and only if, for every possible state of affairs, all instances of q are also instances of p. On the syntactic side, this corresponds to what is usually held for description logics, P subsumes Q if and only if there is no model of Q ∧ ¬P.
2 The Basic Notions
2.1 Essence and Rigidity
A property of an entity is essential to that entity if it must be true of it in every possible situation, i.e., if it necessarily holds for that entity. For example, the property of having a brain is essential to human beings. Every human must have a brain in every possible situation.
A special form of essentiality is rigidity; a property is rigid if it is essential to all its possible instances; an instance of a rigid property cannot stop being an instance of that property in a different situation. For example, while having a brain may be essential to humans, it is not essential to, say, scarecrows in the Wizard of Oz. If we were modeling the world of the Wizard of Oz, the property of having a brain would not be rigid, though still essential to humans. On the other hand, the property being a human is typically rigid, every human is necessarily so.
The fact that we said “typically” in the previous statement requires an immediate clarification. The point of OntoClean is not to help people deciding about the ontological nature of a certain property; this choice depends on the way the domain at hand is conceptualized , and cannot be forced in advance. What OntoClean offers is, rather, a formal framework for expressing (some of) the ontological assumptions lying behind a certain conceptualization (its so-called ontological commitment). Rigidity is the first ingredient of this framework: expressing (by means of meta-properties) whether it holds or not for the relevant properties of our conceptualization helps clarifying the ontological commitment of such conceptualization.
When a property is non-rigid, it can can acquire or lose (some of) their instances depending on the situation at hand. Within non-rigid properties, we distinguish between properties that are essential to some entities and not essential to others (semi-rigid), and properties that are not essential to all their instances (anti-rigid). For example, the property being a student is typically anti-rigid – every instance of student can cease to be such in a suitable situation, whereas the property having a brain in our Wizard of Oz world is semi-rigid, since there are instances that must have a brain as well as others that consider a brain just as a (useful) optional.
Rigidity and its variants are important meta-properties, every property in an ontology should be labeled as rigid, non-rigid, or anti-rigid. In addition to providing more information about what a property is intended to mean, these meta-properties impose constraints on the subsumption relation, which can be used to check the ontological consistency of taxonomic links. One of these constraints is that anti-rigid properties cannot subsume rigid properties. For example, the property being a student cannot subsume being a human if the former is anti-rigid and the latter is rigid. To see this, consider that, if p is an anti-rigid property, all its instances can cease to be such. This is certainly the case for student, since any student may cease being a student. However, no instance of human can cease to be a human, and if all humans would be necessarily students (the meaning of subsumption), then no person could cease to be a student, creating therefore an inconsistency.
2.2 Identity and Unity
Although very subtle and difficult to explain without experience, identity and unity are perhaps the most important notions we use in our methodology. These two things are often confused with each other; in general, identity refers to the problem of being able to recognize individual entities in the world as being the same (or different), and unity refers to being able to recognize all the parts that form an individual entity.
Identity criteria are the criteria we use to answer questions like, “is that my dog?” In point of fact, identity criteria are conditions used to determine equality (sufficient conditions) and that are entailed by equality (necessary conditions).
It is perhaps simplest to think of identity criteria over time (diachronic identity criteria), e.g., how do we recognize people we know as the same person even though they may have changed? It is also very informative, however, to think of identity criteria at a single point in time (synchronic identity criteria). This may, at first glance, seem bizarre. How can you ask, “are these two entities the same entity?” If they are the same then there is one entity, it does not even make sense to ask the question.
The answer is not that difficult. One of the most common decisions that must be made in ontological analysis concerns identifying circumstances in which one entity is actually two (or more). Consider the following example, drawn from actual experience: somebody proposed to introduce a property called time duration whose instances are things like one hour and two hours, and a property time interval referring to specific intervals of time, such as “1:00–2:00 next Tuesday” or “2:00–3:00 next Wednesday.” The proposal was to make time duration subsume time interval, since all time intervals are time durations. Seems to make intuitive sense, but how can we evaluate this decision?
In this case, an analysis based on the notion of identity can be informative. According to the identity criteria for time durations, two durations of the same length are the same duration. In other words, all one-hour time durations are identical – they are the same duration and therefore there is only one “one hour” time duration. On the other hand, according to the identity criteria for time intervals, two intervals of the same duration occurring at the same time are the same, but two intervals occurring at different times, even if they are the same duration, are different. Therefore the two example intervals above would be different intervals. This creates a contradiction: if all instances of time interval are also instances of time duration (as implied by the subsumption relationship), how can they be two instances of one property and a single instance of another?
This is one of the most common confusions of natural language when used for describing the world. When we say “all time intervals are time durations” we really mean “all time intervals have a time duration” – the duration is a component of an interval, but it is not the interval itself. In this case we cannot model the relationship as subsumption, time intervals have durations (essentially) as qualities. More examples of such confusions are provided at the end of this article.
One of the distinctions proposed by OntoClean is between properties that carry an identity criterion and properties that do not. The former are labeled with an ad hoc meta-property, +I. Since criteria of identity are inherited along property subsumption hierarchies, a further distinction is made to mark those properties that supply (rather just carrying) some “own” identity criteria, which are not inherited from the subsuming properties. These properties are marked with the label +O (where O stands for “own”).
Unfortunately, despite their relevance, recognizing identity criteria may be extremely hard. However, in many cases identity analysis can be limited to detecting the properties that are just necessary for keeping the identity of a given entity, i.e., what we have called the essential properties. Obviously, if two things do not have the same essential properties they are not identical. Take for instance the classical example of the statue and the clay: is the statue identical to the clay it is made of? Let us consider the essential properties: having (more or less) a certain shape is essential for the statue, but not essential for the clay. Therefore, they are different: we can say they have different identity criteria, even without knowing exactly what these criteria are. In practice, we can say that “sharing the essential property P,” where P is essential for all the instances of a property Q different from P, is the weakest form of an identity criterion carried by Q. Such criterion can be used to make conclusions about non-identity, if not about identity.
A second notion that is extremely useful in ontological analysis is Unity. Unity refers to the problem of describing the parts and boundaries of objects, such that we know in general what is part of the object, what is not, and under what conditions the object is whole.
Unity can tell us a lot about the intended meaning of properties in an ontology. Certain properties pertain to wholes, that is, all their instances are wholes, others do not. For example, being (an amount of) water does not have wholes as instances, since each amount can be arbitrarily scattered or confused with other amounts. In other words, knowing it is an amount of water does not tell us anything about its parts, and recognizing it as a single entity. On the other hand, being an ocean is a property that picks up whole objects, as its instances, such as “the Atlantic Ocean” is recognizable as a single entity. Of course, one might observe that oceans have vague boundaries, but this is not an issue here: the important difference with respect to the previous example is that in this case we have a criterion to tell, at least, what is not part of the Atlantic Ocean, and still part of some other ocean. This is impossible for amounts of water.
In general, in addition to specifying whether or not properties have wholes as instances, it is also useful to analyze the specific conditions that must hold among the parts of a certain entity in order to consider it a whole. We call these conditions unity criteria (UC). They are usually expressed in terms of a suitable unifying relation, whose ontological nature determines different kinds of wholes. For example, we may distinguish topological wholes (a piece of coal), morphological wholes (a constellation), functional wholes (a hammer, a bikini). As these examples show, nothing prevents a whole from having parts that are themselves wholes (under different unifying relations). Indeed, a plural whole can be defined as a whole that is a mereological sum of wholes.
In OntoClean, we distinguish with suitable meta-properties the properties all whose instances must carry a common UC (such as ocean) from those that do not. Among the latter, we further distinguish properties all of whose instances must be wholes, although with different UCs, from properties all of whose instances are not necessarily wholes. An example of the former kind may be legal agent, if we include both people and companies (with different UCs) among its instances. Amount of water is usually an example of the latter kind, since none of its instances must be wholes (this is compatible with the view that a particular amount of water may become a whole for a short while, e.g., while forming an iceberg. We say that ocean carries unity ( + U), legal agent carries no unity ( −U), and amount of water carries anti-unity ( ~ U).
The difference between unity and anti-unity leads us again to interesting problems with subsumption. It may make sense to say that “Ocean” is a subclass of “Water,” since all oceans are water. However, if we claim that instances of the latter must not be wholes, and instances of the former always are, then we have a contradiction. Problems like this again stem from the ambiguity of natural language, oceans are not “kinds of” water, they are composed of water.
2.3 Constraints and Assumptions
A first observation descending immediately from our definitions regards some subsumption constraints. Given two properties, p and q, when q subsumes p the following constraints hold:
If q is anti-rigid, then p must be anti-rigid
If q carries an identity criterion, then p must carry the same criterion
If q carries a unity criterion, then p must carry the same criterion
If q has anti-unity, then p must also have anti-unity
If q is dependent on property c, then p is dependent on property c
Finally, we make the following assumptions regarding identity (adapted from Lowe ):
Sortal Individuation. Every domain element must instantiate some property carrying an IC ( + I). In this way we satisfy Quine’s dicto “No entity without identity” .
Sortal Expandability. If something is an instance of different properties (for instance related to different times), then it must be also instance of a more general property carrying a criterion for its identity.
Together, the two assumptions imply that every entity must instantiate a unique most general property carrying a criterion for its identity.
3 An Extended Example
In this section we provide a walk-through of the way the OntoClean analysis can be used. This example is based on those presented at various tutorials and invited talks.
We have chosen, following our previous papers, to use a shorthand notation for indicating meta-property choices on classes. Rigidity is indicated by R, identity by I, unity by U, and dependence by D. Each letter is preceded by + , − or ~ , to indicate the positive, negative, or anti meta-property, e.g., being rigid ( + R), carrying an identity criterion ( + I), carrying a common unity criterion ( + U); not rigid ( − R), not carrying an identity criterion ( − I), not carrying a common unity criterion ( − U); being anti-rigid ( ~ R) and having anti-unity ( ~ U). We also used (+O) to indicate when a property carries its own identity criterion, as opposed to inheriting one from a more general property.
3.1 Assigning Meta-Properties
The next step is to assign the meta-properties discussed above to each property in the taxonomy. When designing a new ontology, this step may occur first, before arranging the properties in a taxonomy. Note that the assignments discussed here are not meant to be definitive at all: rather, these represent prima facie decisions reflecting our intuitions about the meaning ascribed to the terms used. The point of this exercise is not so much to discuss the ontological nature of these properties, but rather to explore and demonstrate the logical consequences of making these choices. As we shall see, in some cases they will result contradictory with respect to the formal semantics of our meta-properties, although intuitive at a first sight. In our opinion, this proves the utility of a formal approach to ontology analysis and evaluation.
Everything is necessarily an entity. Our meta-properties assignment is - I - U + R. This is the most abstract property, indeed it is not necessary having an explicit predicate for it.
A location is considered here as a generalized region of space. Our assignment is + O ~ U + R. We assume the property as rigid since instances of locations cannot change being locations. The identity criterion is that two locations are the same if and only if they have the same parts. This kind of criterion is fairly common, and is known as mereological extensionality. It applies to all entities that are trivially defined to be the sum of their parts. It is important to realize that this criterion implies that a location or region cannot “expand” – if so then the identity criteria would have to be different. So, extending a location makes it a different one. So we see that identity criteria are critical in specifying precisely what a property is intended to mean.
3.1.3 Amount of Matter
We conceptualize an amount of matter as a clump of unstructured or scattered “stuff” such as a liter of water or a kilogram of clay. Amounts of matter should not be confused with substances, such as water or clay; an amount of matter is a particular amount of the substance. Therefore, amounts of matter are mereologically extensional, so we assign +O to this property. As discussed above, they are not necessarily wholes, so our assignment is ~ U. Finally, every amount of matter is necessarily so, therefore the property is +R.
What we have in mind here is the property of being a red thing, not the property of being a particular shade color. We see in this case that it is useful to ask ourselves what the instances of a certain property are. Do we have oranges and peppers in the extension of this property, or just their colors? Red entities share no common identity criteria, so our assignment is –I. A common confusion here regarding identity criteria concerns the fact that all instances of red are colored red, therefore we have a clear membership criterion. Membership criteria are not identity criteria, as the latter gives us information about how to distinguish entities from each other. Having a color red is common to all instances of this property, and thus is not informative at all for identity.
A red amount of matter would be an instance of this property, which is not a whole, as would a red ball, which is a whole. Therefore we must choose –U, indicating that there is no common unity criterion for all instances.
Finally, we choose –R since some instances of Red may be necessarily so, and most will not. This weak and unspecific combination of meta-properties indicates that this property is of minimal utility in an ontology, we call them attributions .
We intend here an entity that plays a causal part in some event. Just about anything can be an agent, a person, the wind, a bomb, etc. Thus there is no common identity nor unity criterion for all instances, and we choose –I–U. No instance of agent is necessarily an agent, thus the property is ~ R. Clearly this assignment of meta-properties selects a particular meaning of agent among the many possible ones. See for example  for a discussion on the meaning of causal agent in WordNet.
We see here a group as an unstructured finite collection of wholes. Instances of group are mereologically extensional as they are defined by their members, thus +O. Since, given a group, we have no way to isolate it from other groups, no group is per se a whole, thus ~ U. In any case, like many general terms, Group is fairly ambiguous, and once again this choice of identity criteria and anti-unity exposes the choice we have made. Finally, it seems plausible to assume that every instance of group is necessarily so, thus +R.
3.1.7 Physical Object
We think here of physical objects as isolated material entities, i.e., something that can be “picked up and thrown” (at a suitable scale, since a planet would be considered an instance of a physical object as well…). Under this vision, what characterizes physical objects is that they are topological wholes – so we assign +U to the corresponding property.
For the sake of simplicity, we assume here that no two instances of this property can exist in the same spatial location at the same time. This is an identity criterion, so we assign +O to this property. Note that this is a synchronic identity criterion (see identity and unity, above) – we do not assume a common diachronic identity criterion for all physical objects.
Physical object is a rigid property, so we have +R. To see this, consider the alternative: there must be some instance of the property that can, possibly, stop being a physical object, yet still exist and retain its identity. By assigning rigidity to this property, we assert that there is no such instance, and that every instance of Physical Object ceases to exist if it ceases to be a physical object.
3.1.8 Living Being
Instances of living being must be wholes according to some common biological unity criterion. We do not need to specify it to assign +U to this property.
For identity, it is difficult to assume a single criterion that holds for all instances of living being. The way we, e.g., distinguish people may be different from the way we distinguish dogs. However, a plausible diachronic criterion could be having the same DNA (although only-necessary, since it does not help in the case of clones). Moreover, we can easily think of essential properties that characterize living beings (e.g., the need of taking nutrients from the environment), and this is enough for assigning them + O.
We assume living being to be a rigid property (+R), so if an entity ceases to be living then it ceases to exist. Notice that this is a precise choice that is totally dependent on our conceptualization: nothing would exclude considering life as a contingent (non-rigid) property; by considering it as rigid, we are indeed constructing a new kind of entity, justified by the fact that this property is very relevant for us.
Nothing is necessarily food, and just about anything is possibly food. In a linguistic sense, “food” is a role an entity may play in an eating event. Considering that anything that is food can also possibly not be food, we assign ~ R to this property. We also assume that any quantity of food is an amount of matter and inherits its extensional identity criterion, thus + I and ~ U.
Like for living being, the identity criteria for animal may be difficult to characterize precisely, but we can devise numerous essential properties that apply only to them, or only-sufficient conditions that act as heuristics especially for diachronic identity criteria. Humans, in particular, are quite good at recognizing most individual animals, typically based on clues present in their material bodies. The undeniable fact is that we do recognize “the same” animal over time, so there must be some way that is accomplished. Therefore, we assign + O.
The property is clearly rigid (+R); moreover, being subsumed by living being, it clearly carries unity (+U).
3.1.11 Legal Agent
This is an agent that is recognized by law. It exists only because of a legal recognition. Legal agents are entities belonging to the so-called social reality, insofar their existence is the result of social interaction. All legal systems assign well-defined identity criteria to legal agents, based on, for example, an id number. Therefore, it seems plausible to assign +O. Concerning unity, if we include companies (as well as persons) among legal agents, then probably there is no unity criteria shared by all of them, so we assign − U. Finally, since nothing is necessarily a legal agent, we assign ~ R. For instance, we may assume that a typical legal entity, such as a person, becomes such only after a certain age.
3.1.12 Group of People
A special kind of Group all of whose members are instances of Person. Identity and unity criteria are the same as Group, and thus we have +I ~ U. Finally, we consider Group of People to be rigid, since any entity which is a group of people must necessarily be such.
3.1.13 Social Entity
A group of people together for social reasons. Such as the “Bridge Club” (i.e., people who play cards together). We cannot imagine a common identity criteria for this property, however we assume it is rigid and carries unity. - I + U + R.
Instances of this property are intended to be things like companies, departments, governments, etc. They are made up of people with play specific roles according to some structure. Like people, organizations seem to carry their own identity criterion, and are wholes with a functional notion of unity, so we assign + O + U + R.
We are thinking here of individual fruits, such as oranges or bananas. We assume they have their own essential properties, and can clearly be isolated from each other. Therefore, + O + U + R seems to be an obvious assignment.
This likely adds its on essential properties to those of fruits, so we assign it + O + U + R.
Red apples do not have essential meta-properties in addition to apples. Moreover, no red apple is necessarily red, therefore we assign + I + U ~ R.
This property is actually intended to be vertebrate-animal. This is a biological classification that adds new membership criteria to Animal (has-backbone), but apparently no new identity criteria: + I + U + R.
Like Living Entity and Animal, the Person property is + U + R. It seems clear that specializing from Vertebrate to Person we add some further essential properties, thus we assume that Person has its own identity criteria, and we assign + O.
3.1.20 Butterfly and Caterpillar
Like Animal, Butterfly and Caterpillar have + I + U. However, every instance of Caterpillar can possibly become a non-caterpillar (namely a butterfly), and every instance of Butterfly can possibly be (indeed, must have been) a non-butterfly (namely a caterpillar), thus we assign ~ R to each.
Intuitively, a country is a place recognized by convention as having a certain political status. Identity may be difficult to characterize precisely, but some essential properties seem to be clearly there, so + O. Countries are certainly wholes, so + U. Interestingly, it seems clear that some countries, like Prussia, still exist but are no longer countries, so we must assign ~ R.
3.2 Analyzing Rigid Properties
3.2.1 The Backbone Taxonomy
We now focus our analysis on what we have called the backbone taxonomy, that is, the rigid properties in the ontology, organized according to their subsumption relationships. These properties are the most important to analyze first, since they represent the invariant aspects of the domain. Our sortal expandability and individuation principles guarantee that no element of the domain is “lost” due to this restriction, since every element must instantiate at least one of the backbone properties, that supplies an identity criterion for it.
After making the initial decisions regarding meta-properties and arranging the properties in a taxonomy, we are then in a position to verify whether any constraints imposed by the meta-properties are violated in the backbone. These violations have proven to be excellent indicators of misunderstandings and improperly constructed taxonomies. When a violation is encountered, we must reconsider the assigned meta-properties and/or the taxonomic link. and take some corrective action.
Living beings are not amounts of matter. The first problem we encounter is between Amount of Matter and Living Being. The problem is that a ~ U property cannot subsume one with + U. While it certainly seems to make sense to say that all living beings are amounts of matter, based on the meaning we have assigned there is an inconsistency: every amount of matter can be arbitrarily scattered, but this is certainly not the case for living beings. A further reason against this subsumption link is in the identity criteria: amounts of matter have an extensional identity, that is, they are different if any of their parts is substituted or annihilated – if you remove some clay from a lump of clay, it is a different amount. Living beings, on the other hand, can change parts and still remain the same – when you cut your fingernails off you do not become a different person.
This is one of the most common modeling problems we have. Living beings are constituted of amounts of matter, they are not themselves the matter. Natural language convention fails to capture this subtle distinction, but it is a violation of the intended meaning to claim that all living beings are mereologically extensional.
The solution here is to remove the subsumption link between these two properties, and represent the relationship as one of constitution.
Physical objects are not amounts of matter. Again, we see a violation since a ~ U property cannot subsume one with + U. This is yet another example of constitution being confused with subsumption. Physical objects are not themselves amounts of matter, they are constituted of matter. The solution is to make Physical Object subsumed directly by Entity.
Social entities are not groups of people. Another ~ U + U violation, as well as a violation of identity criteria. Social entities are constituted of people, but, as with other examples here, they are not merely groups of people, they are more than that. A group of people does not require a unifying relation, as we assume these people can be however scattered in space, time, or motivations. On the contrary, a social entity must be somehow unified. Moreover, although both properties supply their own identity criteria, these criteria are mutally inconsistent. Take for instance two typical examples of social entities, such as a bridge club and a poker club. These are clearly two separate entities, even though precisely the same people may participate in both. Thus we would have a situation where, if the social entity was the group of people, the two clubs would be the same under the identity criteria of the group, and different under the identity criteria of the social entity. The solution of the puzzle is that this is, once again, a constitution relationship: a club is constituted by a group of people.
Animals are not physical objects. Although no constraints involving meta-properties are violated in this subsumption link, a closer look at the identity criteria of the two properties involved reveals that the link is inconsistent. Animals, by our account, cease to exist at death, since being alive is an essential property for them. However their physical bodies remain for a time after: being alive is not essential to them. Indeed, under our assumption no physical object has being alive as an essential property. Now, if an animal is a physical object, as implied by subsumption, how could it be that it is at the same time necessarily alive and not necessarily alive? The answer is that there must be two entities, related by a form of constitution, and the subsumption link should be removed.
In this example, it is not the meta-properties, but the methodology requiring to make identity criteria explicit in terms of essential properties that reveals the error.
3.3 Analyzing Non-rigid Properties
Let us now turn our attention to the non-rigid properties, which – so to speak – “flesh out” the backbone taxonomy. In  we have discussed a taxonomy of property kinds based on an analysis of their meta-properties, which distinguishes three main cases of non-rigid properties: phased sortals, roles, and attributions. All these cases appear in our example, and are discussed below.
Among other things, the differences among these property kinds are based on a meta-property not discussed here, based on the notion of dependence. A proper grasping of this notion (which is rather difficult to formalize) is not essential for an introductory understanding of the OntoClean methodology, so we shall rely on intuitive examples only.
3.3.1 Phased Sortals
The notion of a phased sortal was originally introduced by Wiggins . A phased sortal is a property whose instances are allowed to change certain of their identity criteria during their existence, while remaining the same entity. The canonical example is a caterpillar. The intuition here is that when the caterpillar changes into a butterfly, something fundamental about the way it may be recognized and distinguished has changed, even though it is still the same entity. Phased sortals are recognized in our methodology by the fact that they are independent, anti-rigid, and supply identity criteria.
In the typical case, phased sortals come into clusters of at least two properties – an instance of a phased sortal (e.g., Caterpillar) should be able to “phase” into another one (e.g.,Butterfly), and these clusters should have a common subsuming property providing an identity criterion for across phases, according to the sortal individuation principle.
Caterpillars and butterflies. Consider now our example. Caterpillar and Butterfly appear in out initial taxonomy, but there is no single property that subsumes only the phases of the same entity. Our formal analysis shows that there must be such property. After some thinking, we find what we need: it is the property Lepidopteran, which is + O + U + R. This is what supplies the identity criteria needed to recognize the same entity across phases.
Countries. The property Country does not, prima facie, appear to be a phased sortal, yet it meets our definition ( + O ~ R). This is an example where reasoning on the meta-properties assignments and their consequences helps us pushing our ontological analysis further: what are we talking of, here? Is it a region that occasionally becomes a country, and in this case acquires some extra (yet temporary) identity criteria? What happens when something is not a country any more? Does it cease to exist, or does it just undergo the change of a property, like changing from being sunny and being shady? While answering to these questions, we realize we are facing a common problem in building ontologies, that of lumping together multiple meanings of a term into a single property. It seems there are two different interpretations of “country,” one as a geographical region, and another as a geopolitical entity. It is the latter that ceases to exist when the property does not hold any more.
So there are two entities: the Country Prussia and the Geographical Region Prussia. These two entities are related to each other (e.g., countries occupy regions), but are not the same, and therefore we must break the current property into two.
We assign + O + U + R to Country, and + I - U + R to Geographical Region. The intuition is that countries have their own identity criteria, while geographical regions inherit the identity of locations. Countries have clearly a unity, while this is not the case for arbitrary geographical regions. Both properties are now rigid. Interestingly enough, we replaced an anti-rigid property with two rigid properties.
This is a different kind of problem in which subsumption is being used to represent a type restriction. The modeler intends to mean, not that all animals are agents, but that animals can be agents. This is a very common misuse of subsumption, often employed by object-oriented programmers. The correct way to represent this kind of relationship is with a covering, i.e., all agents are either animals or social entities. Clearly this is a different notion than subsumption. The solution is to remove the subsumption links and represent this information elsewhere.
As with the Agent role, being forced to remove these links forces us to reconsider the meaning of the Legal Agent property. A legal agent is simply an entity recognized by law as an agent in some transaction or contract. Again, as with the Agent example, this is not a true subsumption link, but rather another type restriction. The links should be removed and replaced with a covering axiom.
While our notion of what an apple means may seem to be violated by removing the subsumption link to food, the point is that we have chosen to represent the property in a particular way, as a role, and this link is inconsistent with that meaning and should be removed. In this case, the links are probably being used to represent purpose (see, e.g., ), not subsumption.
The final category of properties we consider are attributions. We have one such property in our example, Red, whose instances are intended to be red things. We think that in general it is not useful representing attributions explicitly in a taxonomy, and that the proper way to model attributions is with a simple attribute, like color, and a value, such as red. This quickly brings us to the notion of qualities, discussed in the related chapter of this handbook on Dolce, and we avoid that discussion here.
Attributions do, however, come in handy on occasions. Their practical utility is often found in cases where there are a large number of entities that need to be partitioned according to the value of some attribute. We may have apples and pears, for example, and decide we need to partition them into red and green ones. Ontologically, however, the notion of red-thing does not have much significance, since there is nothing we can necessarily say of red-things, besides their color. This seems to us a very good reason for not consider attributions as part of the backbone. In other words, the backbone taxonomy helps in focusing on the more important classes for understanding the invariant aspects of domain structure, whereas attributions help in organizing the instances on an ad-hoc, temporary basis.
Many people have made useful comments on OntoClean, and have participated in its refinement. We would like to thank in particular Mariano Fernandez Lopez, Aldo Gangemi, Giancarlo Guizzardi, Claudio Masolo, Alessandro Oltramari.
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