GFO Part I Basic Principles
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2.1 Philosophical Assumptions

First we present and discuss our philosophical position. We consider two topics: the notion and ontological status of categories, and the problem of existence. We support a realistic position in philosophy, but there is the need to clarify more precisely the term ``realism''.

There is a close relation between categories and language, hence the analysis of the notion of category cannot be - in our opinion - separated from the investigation of language. Concerning the notion of existence we draw our inspiration partly from Ingarden (32), but mainly from our own ontological investigations and analyses. We use the term entity for everything that exists.

2.1.1 Categories

The discussion in this section is inspired by Jorge Gracia's ideas presented in (23), which proved to be useful for the purpose of conceptual modelling and computer-science ontologies. A general ontology is concerned with the most general categories, with their analysis and axiomatic foundation.

Categories are entities that are expressed by predicative terms of a (formal or natural) language and that can be predicated of other entities. Predicative terms are linguistic expressions which state conditions to be satisfied by an entity. Categories are what predicative terms express, their content and meaning, not the predicative terms themselves, understood as a string of letters in a language. Hence, we must distinguish: the category, the predicative term - as a linguistic entity - expressing the category, and the entities that satisfy the conditions stated by the predicative term.

The predicative term $T$, the expressed category $C$, and the satisfying entity $e$ are mediated by two relations, $\Gexpr(T,C)$ and $\Gsat(C,e)$. We stipulate that a category $C$ is predicated of an entity $e$ if and only if $e$ satisfies the conditions that are associated to $C$. Equivalently we say that an entity $e$ is an instance of a category $C$, or that $e$ instantiates $C$. Hence, we hold that the following three conditions are equivalent: $e$ instantiates $C$, $C$ is predicated of $e$, and $e$ satisfies the conditions of $C$.7 Categories are designated and expressed by terms of a language. Terms of a language are words, sentences, texts, i.e., every expression that is well-formed according to the grammatical rules of the language.8

We assume that categories are conceived in such a way that we are not forced to commit ourselves to realism, conceptualism, or nominalism (23). This assumption is compatible with our pluralistic approach discussed in the introduction above and it seems to be the most adequate for the purpose of computer-science ontologies and conceptual modelling. According to the approach of (23) we derive several kinds of categories from basic philosophical assumptions. We restrict these to the following basic kinds of categories: immanent categories (also called in the following universals), concepts (conceptual structures) , and symbolic structures. Immanent categories are not outside the world of human experience, but are constituents of this world. Concepts are categories that are expressed by linguistic signs and are present in someone's mind. Symbolic structures are signs or texts that may be instantiated by tokens. There are close relations between these three kinds of categories: an immanent category is captured (grasped) by a concept which is denoted (designated) by a symbolic structure. Texts and symbolic structures may be communicated by their instances that are physical tokens.

An important problem in conceptual modelling is to present (specify) categories in a formal modelling language, and to determine which conditions a formal language should satisfy to capture categories of several kinds adequately.

Sets play a particular role in GFO. We hold that a set cannot be predicated of its members, but there are, of course, specifications of sets expressing categories which can be predicated of sets.9For this reason we do not consider sets as categories. Sets serve as a formal modelling tool and are associated to the abstract top level of GFO.

2.1.2 Existence and Modes of Being

In (32) a classification of modes of existence is discussed that is useful for a deeper understanding of entities of several kinds. According to (32) there are - roughly - the following modes of being: absolute, ideal, real, and intentional entities. This classification can be to some extent related to Gracia's approach and to the levels of reality in the spirit of Nicolai Hartmann (29). But, the theory of Roman Ingarden is not sufficiently elaborated compared with Hartmann's large ontological system. For Ingarden there is the (open) problem, whether material things are real spatio-temporal entities or intentional entities in the sense of the later Husserl. We hold that there is no real opposition between the realistic attitude of Ingarden and the position of the later Husserl, who considers the material things as intentional entities being constructed by a transcendental self. Both views provide valuable insights in the modes of being that can be useful for conceptual modelling purposes.

Robert Hoehndorf 2006-10-18


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