An examination of the ontology in its philosophical and Knowledge Representation senses
‘The essential nature of the propositional sign becomes very clear when we imagine it made up of spatial objects (such as tables, chairs, books) instead of written signs.’ (Wittgenstein 1922 sec. 3.1431)
Introduction
In Chapter 2 we presented the Semantic Web as a solution to the problem of heterogeneous data within archaeology. Using RDF, we showed that it was possible to produce archaeological content in a form that would allow it to be embedded within an existing mesh of domain knowledge. The argument being that this linked nature of the data increases its inherent value and as a consequence the value of the wider pool of the knowledge. SPARQL was presented as a route into what can appear on first inspection to be a largely unintelligible mass of information. It simplifies the querying process, attenuating its complexity and allowing individual nodes of information to be plucked from vast graphs of networked knowledge. We also made the apparently logical observation that data that is meaningful is more valuable to the research community than data that is absent of meaning, and RDF and SPARQL need to be understood as avenues into this meaning. However, we have yet to consider the nature of this meaning and how this meaning might be injected into an RDF dataset and subsequently extracted by a machine agent. The answer to this question is found in the form of what is known as a Knowledge Representation (KR) ontology and this chapter will show how essential the ontology has become to the practical workings of the Semantic Web.
To view the Semantic Web ontology simply as a practical solution to a functional challenge would, however, be a mistake. The study of ontologies is a central question in philosophy with its origins being traced back to the beginnings of when humans started to ask deeper questions about themselves and about the nature of their relationship with the world around them. Therefore, to say that a Semantic Web ontology gives meaning to an RDF dataset and that this satisfies the ontological matter would be to entirely
misrepresent exactly what the Semantic Web is attempting to achieve; that is to represent views onto aspects of reality, whatever those interpretations might be.
To give some context to this understanding of the ontology, this chapter will begin by asking what an ontology is in its broadest sense. This history will begin at the advent of the Western philosophical tradition in the teachings of the Greek Presocratic philosophers and end with the ontology’s employment as a practical tool of KR scientists in the modern era. We will then look at some of the most important ontologies currently in use within the Archaeological Semantic Web. The CIDOC CRM and its CRM-EH and CRMarchaeo extensions, whose historical trajectories were introduced in the last chapter, are deconstructed and explained from the perspective of both the Semantic Web data publisher and consumer.
Why the philosophical perspective?
The OED offers the following definition for the term ‘ontology’. It is a ‘system similar in scope to modern predicate logic, which attempts to interpret quantifiers without assuming that anything exists beyond written expressions’. This understanding of ontology would seem to fit well with this thesis’s focus. It sees the ontology as a tool used within the study and application of logic and the Semantic Web is, if nothing else, concerned with the formulation of logical rules and the outcomes of their application. The OED, however, also offers a second definition. In this case, the ontology is described in its philosophical sense. It states that ontology is ‘the science or study of being; that branch of metaphysics concerned with the nature or essence of being or existence’. On first inspection, this presents quite a different understanding to the previous statement. If we want to approach a fuller understanding of the ontology in the context of its use within the Semantic Web model, is it necessary that we also attempt to understand what on the face of it is a far more complex and broad philosophical range of issues? And given that this is not a philosophical treatise, is this a valid use of our time? After all, the majority of works that deal with the Semantic Web do so from a primarily, if not exclusively, functionalist point of view.
While valid arguments can be made to the contrary, I would contend that it is worthwhile to pursue this generally neglected subject, albeit in a necessarily concise fashion. As was stated in the introductory chapter, a primary objective of this thesis is that it be written from a reflexive point of view, insofar as is possible. The first chapter also expressed the
hope that the reader of this work be open to this particular way of engaging with the material. In that context, it would be disingenuous to now ignore this aspect of the subject, particularly given that it will prove fundamental to our later study of the wider implications of the Archaeological Semantic Web.
Where to begin?
Karl Popper said that there are two fundamental questions, which a person can ask. The first is, what is the nature of the world around us and the second, how can we obtain knowledge about that world (Popper 2012 p. 8)? And so, in Popper’s eyes, philosophy can be broken down into two basic categories of enquiry, which we call ontology and epistemology. It is not difficult to see that there is a connection and dependence between these two lenses of enquiry. Epistemology, for instance, cannot exist without an object to study and the abstract sense of these entities is agreed upon by an ontology. Therefore, we can say that epistemology relies upon there being a particular model of being on which to operate and in that sense ontology precedes epistemology. However, in practice the two are tightly bound together and it is difficult to consider one in isolation of the other. As such, this following account may include passages that address epistemological as well as ontological concerns.
If we are to tackle this subject of the philosophical ontology, then we need a starting point. Unfortunately, but perhaps predictably, there is no neat point in time, which can be found to satisfy this need. It is highly likely that humans have been preoccupied with matters of their own existence and their place within the wider world for as long as they have had the mental capacity to do so. As a result, the true chronology of ontological thought is undoubtedly a very deep one (Lewis-Williams 2002). However, we will resist the temptation of delving down that particular rabbit hole and instead begin our journey on the steadier intellectual footing of the first Western philosophical thinkers, in the late 6th and 5th Centuries BCE philosophy of Magna Graecia.
Thales is widely acknowledged to be the first philosopher to espouse a set of principles, which we now define as Presocratic (Mansfeld 1985). And, as is the case for most of this collection of early scholars, what we know of his work comes to us indirectly through the writings of other philosophers, most notably the 4th Century BCE Aristotle and his student, Theophrastus (McDiarmid 1953 p. 88). While not ideal, and while we need to be aware of the distinct possibility that many of these accounts will be effected by more
than a small degree of contamination, they are all the evidence that we have in many cases and, therefore, we must make the best of them (Cherniss 1951 p. 319; Waterfield 2000 pp. xiii–xiv). Thales’s cosmology paints a picture of an earth resting upon a vast body of water, as a ship floats upon the sea (DK 11A14-5).61 Continuing this aquatic theme, Thales also believed that all matter comes from water (DK 11A12) and because of this Aristotle felt that Thales anticipated his own material cause of being axiom; this stated that all entities had a material cause (Lloyd 2012 p. 1). Thales also believed that the being was divided between the soul on the one hand and the external material form on the other (DK 11A22).
Anaximander, a fellow Milesian and a near contemporary of Thales, argued for a different reading of the universe’s form, although Popper argues that his is likely to be a critical reaction against the earlier philosopher’s model of existence (Popper 2012 p. 10). Anaximander’s worldview is of an earth that is shaped like a drum (DK 12A10) with two opposite-facing surfaces. This hypothesis appears to anticipate the science of gravity and relativity when he states that ‘we walk on one of its flat surfaces, while the other is on the opposite side’ (DK 12A145). Anaximander and the other Presocratics lived in a time of great public and private transformation in the Greek world (Osborne 2007). While we need to be on our guard against overstating the effect of these changes, it is clear that the great minds of the Greek city states were struggling with the challenge of charting a course from a point in which knowledge was either divine and unobtainable or mortal and knowable to a point where knowledge creation was firmly in the hands of humans or as Finley puts it, ‘it is as if the early mist had risen to uncover no longer a world of gods but a bright mid-morning’ (Finley 1966 p. 80). Anaximander’s great contribution was to begin a process of rationalising the nature of the relationship between divine and mortal beings on the one hand and divine and mortal knowledge on the other. And he attempted to do so by proposing hypotheses that might explain these concepts within the context of a fairly rigid set of permissible parameters. His ‘apeiron’ concept can be seen in this context (Waterfield 2000 p. 5). It is an infinite and intransient entity from which all being emerges and to which all being returns (DK12A9). Anaximander expands upon the singularity of Thales’s water model by stating that the apeiron is also the source
61 The DK abbreviation refers to Diels and Kranz (1951). This is a compendium of many of the
of opposites (cold and hot, rough and smooth, et cetera) and that these introduce variety into beings.
A key figure in this period is the Elean, Parmenides, whose ontological proposition bucks the general trend towards seeing change as a fundamental aspect of reality.62 The only surviving work accredited to him is a type of sacred poem, which is now separated by experts in the field into three units: a prologue, a section called ‘The Way to Truth’ and a final section entitled, ‘The Way of Appearance’ (Waterfield 2000 p. 49). The conceit of the poem is a journey taken by the narrator from the land of mortals down into the underworld.63 While the events of the piece are structured around this physical expedition, the real journey is a metaphorical one from a position of ignorance to an understanding of how things really are. This knowledge is revealed to the self-proclaimed mystic, Parmenides, by an unnamed goddess, who offers Parmenides a choice of two paths to the way of truth, but the inference is that only one of these options holds the possibility of obtaining that goal (DK 28B2). Presumably, this implies that humans have up until now erred in their appreciation of exactly what and how the world is. To say that Parmenides’s poem veers on the side of opaqueness would be an understatement and as a result the intended meaning encoded within its narrative has attracted vigorous comment and counter-comment over the years.
While it would be convenient in the context of this study of ontology to see ‘The Way to Truth’ as a treatise on Parmenides’ position on ontology and ‘The Way of Appearance’ as the poet’s views on epistemology, the reality is that there is no such clear distinction between the two topics and in that sense we must be careful when reading the first half of the poem as an unconditional ontological work. Having said that, we can definitely use the poem in its entirety to attempt an outline in broad terms a Parmenidean ontological model. The goddess’s role is to guide Parmenides in what it means to say that something ‘is’ (DK 28B6). She explains that there is a difference between thinking that something exists and for that something to actually exist (DK 28B3). There is also the problem of what is not. This conceptualisation of negation or absence has troubled philosophers
62 Popper notes how the period can be broadly defined by a movement away from an ontological
model which saw stasis as the norm, as is testified in the writings of Homer, to a cosmology of constant change by the time of Aristotle (Popper 2012 p. 14).
63 See Kingsley for an alternative shamanic take on the meaning of Parmenides’s journey
ever since (Azzouni 2012; Crane 2012) and, while the intent of the passages that deal with this subject are far from straightforward, the contribution of Parmenides is mainly that he is voicing these concerns for the first time. The poem also alludes to the fact that in order for an entity to exist, it must have attributes associated with it. Parmenides also learns that all entities are immune to death and are unchanging both in terms of space and time, which is a critical insight given the overall context of ontological stasis (Waterfield 2000 p. 53).
The Pythagoreans, an ascetic group of philosophers, who were almost cult-like in their devotion to a particular way of thinking and way of life, are named after their founder Pythagoras, a Greek Samian, who emigrated to the south coast of Italy, possibly fleeing the persecution of the tyrant, Polycrates (Waterfield 2000 p. 89). Unfortunately, very little of the work of Pythagoras or his followers has survived and we are left with the familiar problem of unpicking these original beliefs from the tangle of later references, in this case mainly Plato’s (Riedweg 2005 pp. 116–118). However, it does appear to be the case that Pythagoras held an ontological viewpoint that diverged markedly from the materialist view espoused by the Milesians. The Pythagoreans held that the ultimate reduction of all being was numeric (Aristotle fr.203 Rose). In that sense, the characteristics that applied to numbers – being even or odd, their limits and so on – could all be applied to things, as well. Aristotle suggests that limits for the Pythagoreans are, therefore, in some way fundamental to the nature of a thing (Waterfield 2000 p. 91). Unsurprisingly, mathematics in the form of arithmetic is a key analytical tool for the Pythagoreans. It is not simply used to solve abstract problems but is instead a means by which the true reality of being might be uncovered (Nidditch 1983 p. 12). In this light, the Pythagoreans advocated a cosmology known as the ‘The Harmony of the Spheres’, which relates the universe’s structure to the mathematical rules governing sound and music and which proved immensely popular as a model for later cosmologists (Kahn 2001 p. 166).
Platonic and Aristotelian ontological models
By the time of Plato and his pupil, Aristotle’s arrival on the scene, a number of quite different models for how the world is and how it reveals itself had already been proposed. The era of the Presocratics had been characterised not by a dogmatic uniformity of opinion often associated with the idea of a school of thought in the modern sense but by a willingness to critique the established position (Popper 2012 p.
10). Socrates, Plato and Aristotle continued this tradition as they proposed their own ontological models and it is within their work that we can begin to clearly identify the basis of an ontology that is recognisable in the modern KR sense.
Plato’s primary contribution to the ontological debate was his ‘Theory of Forms’ (Dancy 2001). The Theory of Forms or ‘Theory of Two Worlds’, as it is also labelled, defines a bipartite model of reality and this is introduced most famously in the philosopher’s Allegory of the Cave (Fogelin 1971). On the one hand, there is the real world that is occupied by entities known as forms and then there is the world of illusion, which we, as humans, inhabit. Plato’s ontological model is very much predicated on the idea that the real forms cannot be accessed or known directly (Cooper 1970). Only representations of them, which are illusionary and, ultimately not truth, can be accessed. In this sense, Plato’s ontology presents a fundamental epistemological problem that apparently cannot be overcome. Theoretically, it is entirely independent of knowledge but it can only be understood as an object of epistemological agency (Calvert 1970 p. 41).
Plato’s forms and the illusionary ‘particulars’ that we perceive are related and there is interplay between the two. It is tempting to view the Platonic form as an ideal prototype for its illusionary counterpart. While the forms are universal and never come into or out of being, the particulars change, are born and decay and go out of existence. Lacewing (2007) illustrates this point by considering an example where an object shows reflections of beauty. The object may die and, therefore, go out of existence but Plato would argue that the fact that the beauty form does not also die and that other particulars can be seen to reflect this form henceforth, shows that the forms and their reflections are not tied to one another.
Aristotle, for his part, argues against this two-world reality dichotomy (Fine 1993). For Aristotle, there is only one world of reality and that is the natural world (Lavine 1985 pp. 70–71). He attacks the Platonic Theory of Forms on the following basis. First, he turns on its head the Platonic idea that knowledge comes from the unchanging real world of the forms. For Aristotle, the unchanging world is the natural world, which is made up of what he calls substances and these are the source of knowledge. He applies his Aristotelian logic to find that Platonic forms must be merely copies of real concrete
things and that the two-world model is unsustainable because it makes it impossible to explain how the sensible and intelligible things might be related.
The ontology of Aristotle is, like Plato’s, tightly bound to his epistemology. To know, for Aristotle, is to understand the causes of a substance and in his ‘Four Causes’ theory, the Aristotelian ontology is defined as a function of its unveiling via his epistemological