A major and seemingly endless debate (feud?) within the ontology world is between those who describe themselves as realists, and those who I’ll call conceptualists (I’m not sure many would use this label, but the people i have in mind are united by a love of the word ‘concept’.)
Ever since I encountered this argument, I’ve thought of myself as taking a realist approach to ontology building (at least for ontologies that cover scientific knowledge). My reasons for this haven’t really changed much in the face of the various arguments I’ve encountered:
Science makes claims about the world and assumes that world to be consistent. We are trying to build logically consistent structures that are useful for making true inferences – making yet more claims about the world that logically follow from the claims made by science. The results of those inferences will be judged by how they match reality. An inference that is demonstrably false indicates a problem with the initial assertions (or with the inference mechanism). All of this gives us a clear mechanism for judging the quality of an ontology: according to current scientific understanding, are the assertions and inferences of an ontology true?
It seems reasonable to me to claim that, if I build an anatomy ontology that can answer the question “What bones are part of normal human arm?” I’m recording assertions about actual (real) anatomy . The ontology is asserting that – if you were to dissect a normal* human arm, you’d find a set of bones with some specific set of characteristics defined in the ontology. To put it another way, the ontology that can answer this question is recording some regularity in the world. It is a statement about the existence of some class of structures. (* to be fair, the term ‘normal’ here is a good place to start challenging the claims I’m making).
The assertions made in an ontology may, of course, be wrong. But it would be rather silly to claim that infallibility is required in order to follow a realist approach. Only caricature realists (Mr Gradgrind?) believe science to be infallible. To say that we have reason to believe the claims science makes about reality because they are based on evidence is not to say that science’s claims are infallible. Scientists spend their lives judging whether particular claims are reasonable to believe given the evidence. In many cases, the evidence is so overwhelming that we might go further and say that the claims in question are beyond reasonable doubt.
But, who can discount new evidence coming along at some point that overturns such theories? Well, as a realist I think that if a theory is convincingly overturned, we should amend our ontologies accordingly. What would a non-realist do? Should a conceptualist’s ontology of chemistry include phlogiston?
In the meantime, I think we should just record evidence for any assertion we make that might be controversial and link our assertions to the literature that supports them. I see no reason not to extend this to widely believed theoretical predictions. One could make a term for the class Higgs Boson and populate it with predicted properties. I don’t see this as anti-realist: the prediction is built on much indirect evidence as well as theory. Experiments at the LHC should soon tell us whether to delete the term, or to associate more evidence and literature with it.
A realist approach distinguishes what I do clearly from those who think the job of ontologies is to model the language that scientists use. Even within small expert communities of scientists, very many terms are not used consistently. Partly this is because scientists learn many words in the same way as the rest of us – not by memorising some definition but by following the example of others. When more precision is needed for an argument, this is usually specified by the context of usage of a term. But ontologies and ontology annotation, rip words from their context. While we use words from scientific discourse and try to reflect their meaning as closely as possible, in the end the meaning of an ontology term comes from its definition. And to make ontologies that are useful for reasoning, this definition is almost inevitably tighter than the various meanings of general usage
So why are so many intelligent people hostile to a realist approach?
One possible reason is a failure of nerve. Many people become quite nervous at talk of truth and reality. They want to build their epistemic uncertainty into their ontologies. Why anyone should consider this a practical approach has never been clear to me.
Perhaps as important as any direct reason are the various things associated with the approaches of some prominent figures in the realist camp.
Perhaps foremost among these is an insistence by certain realists on the existence and importance of Universals. Some would say that only Universals should be allowed in an ontology. But what’s a universal? It could be an Aristotelian natural type. (Anyone who knows some history of biology should be nervous of this). But I see the term simply as a way of talking about the regularities in the world that are both the objects of scientific discourse and the basis of scientific generalizations. This excludes completely arbitrary groupings. A class that includes only laptops and empty coffee cups is unlikely to be the subject of scientific generalizations. The class of benzene molecules is. However, the grey area between these two extremes is large. I suspect that trying to pre-judge some crisp boundary between the two is rather a waste of time and could all too easily end up excluding classes scientists do find it useful to generalize about.
Finally, some realists have an attachment to particular upper ontologies that a number of conceptualists object to – perhaps with good reason. But, given that realists themselves often disagree vigorously about upper ontologies, objection to some particular upper ontology is not necessarily an objection to realism.
So, for now at least, I’m convinced that realism provides a practical approach to building ontologies of science. But, I’d be very interested to hear arguments against those I’ve presented here.