Opening plenary talk: Recent advances in metaphysics

In some quarters, the expression 'advance in metaphysics' may sound like a contradiction in terms, a widespread assumption being that progress is never really made in philosophy, in the way that it is in the empirical sciences. But a discipline need not be empirical in order to be capable of progress, as the case of mathematics demonstrates. Even so, metaphysics may be thought to compare unfavourably with mathematics, because there is far less consensus amongst its practitioners than there is amongst mathematicians. This, however, is unsurprising, given that metaphysics addresses the most difficult questions that confront the human mind, concerned as it is with the fundamental structure of reality as a whole. Central to that concern is ontology, the science of being. And central to ontology is category theory. This is an area of philosophy in which advance most certainly has been made in recent years and one which has much potential for fruitful practical applicationOntological categories are categories of being, not categories of thought. Ontology is the science of being, not the science of our thought about being. Many philosophers and non-philosophers are tempted to think otherwise because they are consciously or unconsciously influenced by the pervasive relativism and anti-realism of the current intellectual climate. They are persuaded that we can know nothing about the structure of reality 'as it is in itself,' and so must retreat to contemplation of the structure of our thoughts about reality. This is a position that is at once defeatist and internally inconsistent. If we can legitimately aspire to any sort of knowledge at all, we can and must aspire to some knowledge of the structure of reality. At the same time, we must forgo any claim to infallibility in this or any other branch of knowledgeOntological categories are differentiated by the distinctive existence- and identity-conditions of their members. A fundamental ontological category is one whose members have existence- and identity-conditions which are not exhaustively specifiable in terms of ontological dependency relations between those members and the members of other ontological categories. It is a matter of debate how many, and which, fundamental ontological categories should be recognised. Some contemporary metaphysicians favour an ontology which recognises only one fundamental ontological category, that of so-called 'tropes,' or individual property- and relation-instances. Others favour a two-category ontology comprising, on the one hand, properties and relations conceived as universals and, on the other, individual bearers of properties, that is, individual 'objects' or 'substances.' A third option is a four-category ontology which recognises as fundamental two categories of particulars (individual property- and relation-instances, and individual substances) and two categories of universals (properties and relations, and substantial kinds). Each of these rival ontologies has some claim to provide the best metaphysical foundation for natural science. What cannot be disputed is that recent debate between their adherents has been healthy and has led to genuine progress in our attempts to grasp the fundamental structure of reality.