There is a widespread assumption amongst non-philosophers, which is shared by a good many practising philosophers too, that ‘progress’ is never really made in philosophy, and above all in metaphysics. In this respect, philosophy is often compared, for the most part unfavourably, with the empirical sciences, and especially the natural sciences, such as physics, chemistry and biology. Sometimes, philosophy is defended on the grounds that to deplore the lack of ‘progress’ in it is to misconceive its central aim, which is to challenge and criticise received ideas and assumptions rather than to advance positive theses. But this defence itself is liable to be attacked by the practitioners of other disciplines as unwarranted special pleading on the part of philosophers, whose comparative lack of expertise in other disciplines, it will be said, ill-equips them to play the role of all-purpose intellectual critic. It is sometimes even urged that philosophy is now ‘dead’, the relic of a pre-scientific age whose useful functions, such as they were, have been taken over at last by genuine sciences. What were once ‘philosophical’ questions have now been transmuted, allegedly, into questions for more specialised modes of scientific inquiry, with their own distinctive methodological principles and theoretical foundations.

This dismissive view of philosophy is at once shallow and pernicious. It is true that philosophy is not, properly speaking, an empirical science, but there are other disciplines of a non- empirical character in which progress most certainly can be and has been made, such as mathematics and logic. So there is no reason, in principle, why progress should not be made in philosophy. However, it must be acknowledged that even professional philosophers are in much less agreement amongst themselves as to the nature of their discipline and the proper methods of practising it than are mathematicians and logicians. There is more disagreement about fundamentals in philosophy than in any other area of human thought. But this should not surprise us, since philosophy is precisely concerned with the most fundamental questions that can arise for the human intellect.

The conception of philosophy that I favour is one which places metaphysics at the heart of philosophy and ontology — the science of being — at the heart of metaphysics.¹ Why do we need a ‘science of being’, and how is such a science possible? Why cannot each special science, be it empirical or a priori, address its own ontological questions on its own behalf, without recourse to any overarching ‘science of being’? The short answer to this question is that reality is one and truth indivisible. Each special science aims at truth, seeking to portray accurately some part of reality. But the various portrayals of different parts of reality must, if they are all to be true, fit together to make a portrait which can be true of reality as a whole. No special science can arrogate to itself the task of rendering mutually consistent the various partial portraits: that task can alone belong to an overarching science of being, that is, to ontology. But we should not be misled by this talk of ‘portraits’ of reality. The proper concern of ontology is not the portraits we construct of it, but reality itself.

Here, however, we encounter one of the great divides in philosophy, whose historical roots lie in the seventeenth and eighteenth centuries. There are those philosophers — Kant is the most obvious and seminal figure — who consider that we cannot, in fact, know anything about reality ‘as it is in itself’, so that ontology can be coherently conceived only as the science of our thought about being, rather than as the science of being as such. On the other hand, there are philosophers, many of whom would trace their allegiances back to Plato and Aristotle, who think that there is no obstacle in principle to our knowing at least something about reality as it is in itself. On behalf of this view, which I share, it may be urged that to deny the possibility of such knowledge is ultimately incoherent and self-defeating. The easiest way to sustain this charge is to point out that if, indeed, we could know nothing about reality as it is in itself, then for that very reason we could know nothing about our own thoughts about, or portrayals of, reality: for those thoughts or portrayals are nothing if not parts of reality themselves. In short, ontological questions — understood as questions about being rather than just about our thoughts about being — arise with regard to the ontological status of our thoughts, and of ourselves as thinkers of those thoughts: so that to attempt to recast all ontological questions as questions about our thoughts about what exists is to engender a regress which is clearly vicious.

This still leaves unanswered the question of how we can attain knowledge of being, or of reality ‘as it is in itself’, especially if ontology is conceived to be not an empirical but an a priori science. The answer that I favour divides the task of ontology into two parts, one which is wholly a priori and another which admits empirical elements. The a priori part is devoted to exploring the realm of metaphysical possibility, seeking to establish what kinds of things could exist and, more importantly, co-exist to make up a single possible world. The empirically conditioned part seeks to establish, on the basis of empirical evidence and informed by our most successful scientific theories, what kinds of things do exist in this, the actual world. But the two tasks are not independent: in particular, the second task depends upon the first. We are in no position to be able to judge what kinds of things actually do exist, even in the light of the most scientifically well-informed experience, unless we can effectively determine what kinds of things could exist, because empirical evidence can only be evidence for the existence of something whose existence is antecedently possible.

This way of looking at ontological knowledge and its possibility demands that we accept, whether we like it or not, that such knowledge is fallible — not only our knowledge of what actually does exist, but also our knowledge of what could exist. In this respect, however, ontology is nowise different from any other intellectual discipline, including mathematics and logic. Indeed, it is arguable that it was the mistaken pursuit of certainty in metaphysics that led Kant and other philosophers in his tradition to abandon the conception of ontology as the science of being for a misconception of it as the science of our thought about being, the illusion being that we can attain a degree of certainty concerning the contents of our own thoughts which eludes us entirely concerning the true nature of reality ‘as it is in itself’.