Cleaning-up WordNet's Top-Level

ion_1 Attribute Color Chromatic_Color Measure$Quantity$Amount$Quantum Relation_1 Set_5 Space_1 Time_1 Act$Human_Action$Human_Activity Action_1 Activity_1 Forfeit$Forfeiture$Sacrifice Entity$Something Anticipation Causal_Agent$Cause$Causal_Agency Cell_1 Inessential$Nonessential Life_Form$Organism$Being$. Object$Physical_Object Artifact$Artefact Edge_3 Skin_4 Opening_3 Excavation$. Building_Material Mass_5 Cement_2 Bricks_and_Mortar Lath_and_Plaster Body_Of_Water$Water Land$Dry_Land$Earth$. Location Natural_Object Blackbody_Full_Radiator Body_5 Universe$Existence$Nature$. Paring$Parings Film Part$Portion Body_Part Substance$Matter Body_Substance Chemical_Element Food$Nutrient Part$Piece Subject$Content$Depicted_Object Event_1 Fall_3 Happening$Occurrence$Natural_Event Case$Instance Time$Clip Might-Have-Been Group$Grouping Arrangement_2 Biological_Group Citizenry$People Phenomenon_1 Consequence$Effect$Outcome. Levitation Luck$Fortune Possession_1 Asset Liability$Financial_Obligation$. Territory$Dominion$. Transferred_Property$. Psychological_Feature Cognition$Knowledge Structure Feeling_1 Motivation$Motive$Need State_1 Action$Activity$Activeness Being$Beingness$Existence Condition Damnation$Eternal_Damnation Figure 1. WordNet's Top Level. Someone could argue that every person is necessarily a causal agent, since "agentivity" (capability of performing actions) is an essential property of human beings. In this case Causal_ Agent would be intended as a synonym of "intentional agent", and would be considered as rigid. But in this case it would have only hyponyms denoting things that are (essentially) causal agents, including animals, spiritual beings, the personified Fate, and so on. Unfortunately, this is not what happens in WordNet: Agent, one of Causal_Agent hyponyms, is defined as: "an active and efficient cause; capable of producing a certain effect; (the research uncovered new disease agents)". Causal_Agent subsumes roles such as Germicide, Vasoconstrictor, Antifungal. Instances of these concepts are not causal agents essentially. This means that considering Causal_Agent as rigid would introduce further inconsistencies. These considerations allow us to add a pragmatic guideline to our methodological techniques: when deciding about the formal meta-property to attach to a certain concept, it is useful to look at all its children. 3.2. 4 Heterogeneous levels of generality Going down the different layers of WordNet's top level, we register a certain "heterogeneity" in their intuitive level of generality. For example, among the hyponyms of Entity there are types such as Physical_Object, and roles such as Subject. The latter is defined as "something (a person or object or scene) selected by an artist or photographer for graphic representation", and has no hyponyms (indeed, almost any entity can be an instance of Subject, but none is necessarily a subject). For Animal (subsumed by Life_Form) this heterogeneity becomes clearer. Together with classes such as Chordate, Larva, Fictional_Animal, etc., we find out more specific concepts, such as Work_Animal, Domestic_Animal, Mate_3, Captive, Prey, etc. We are induced to consider the formers as types, while the latters as roles. In short, intuitively some synsets sound too specific when compared to their siblings. Look at them from the formal point of view we are developing, we can pinpoint their "different generality" by means of the distinction between types and roles. 4 The OntoClean preliminary top-level Let us now introduce below the top-level categories that we have used for our experiment, each one characterized by the meta-properties discussed above. They represent a first draft of the top-level distinctions we plan to use in our OntoClean methodology. They have been chosen in order to be as general and neutral as possible, although (differently from the formal properties used to characterize them) they reflect our cognitive bias, aimed at capturing the ontological categories lying behind natural language and human commonsense. For the time being, we have also avoided to impose a strong taxonomic structure to these categories, focusing on producing a more or less flat list of basic concepts. In the future, a further restructuring will be probably needed (for instance to better account for the continuants/occurrents distinction, which we have only marginally addressed here). However, the nature of the categories described below would not change. All categories listed below are considered to be rigid, as they are assumed to reflect essential properties of their instances. This essentiality is the result of our commonsense conceptualization: when an amount of matter is considered as forming an object, the decision to consider this "objecthood" as an essential property is only the result of our conceptualization. 4.1 Aggregates (~D, ~U) The common trait of aggregates is that they are independent entities, and none of them is an essential whole. This means that the corresponding property carries anti-unity (~U). We consider two kinds of aggregates: Amounts of matter and Arbitrary collections. The latter can be also called groups, or perhaps sets; we prefer however to use set for abstract entities, and group does sometimes denote something with an intrinsic unity. Arbitrary collections are just mere sums of wholes which are not themselves essential wholes (like the sum of a person’s nose and a computer keyboard). Amounts of matter are extensional (+E), in the sense that they change their identity when they change some parts; arbitrary collections can be considered as pseudo-extensional, in the sense that they change their identity when a member is changed, while a change in the parts of a member may be allowed. 4.2 Objects (~D, *U) The main characteristic of objects is that all of them are independent essential wholes. This does not mean that the corresponding property (being an object) carries +U, since there is no common unity criterion for objects. Among objects, we distinguish between extensional bodies and ordinary objects. Bodies are considered to be extensional (+E), while ordinary objects are not (~E). This means that we assume that all ordinary objects can change some of their parts while keeping their identity. 7 We can draw similar observations for relation_1 and set_5 with respect to abstraction_1, etc.