See VOLUNTARISM. metaphysics, most generally, the philosophical investigation of the nature, constitution, and structure of reality. It is broader in scope than science, e.g., physics and even cosmology (the science of the nature, structure, and origin of the universe as a whole), since one of its traditional concerns is the existence of non-physical entities, e.g., God. It is also more fundamental, since it investigates questions science does not address but the answers to which it presupposes. Are there, for instance, physical objects at all, and does every event have a cause? So understood, metaphysics was rejected by positivism on the ground that its statements are ‘cognitively meaningless’ since they are not empirically verifiable. More recent philosophers, such as Quine, reject metaphysics on the ground that science alone provides genuine knowledge. In The Metaphysics of Logical Positivism (1954), Bergmann argued that logical positivism, and any view such as Quine’s, presupposes a metaphysical theory. And the positivists’ criterion of cognitive meaning was never formulated in a way satisfactory even to them. A successor of the positivist attitude toward metaphysics is P. F. Strawson’s preference (especially in Individuals, 1959) for what he calls descriptive metaphysics, which is ‘content to describe the actual structure of our thought about the world,’ as contrasted with revisionary metaphysics, which is ‘concerned to produce a better structure.’ The view, sometimes considered scientific (but an assumption rather than an argued theory), that all that there is, is spatiotemporal (a part of ‘nature’) and is knowable only through the methods of the sciences, is itself a metaphysics, namely metaphysical naturalism (not to be confused with natural philosophy). It is not part of science itself. In its most general sense, metaphysics may seem to coincide with philosophy as a whole, since anything philosophy investigates is presumably a part of reality, e.g., knowledge, values, and valid reasoning. But it is useful to reserve the investigation of such more specific topics for distinct branches of philosophy, e.g., epistemology, ethics, aesthetics, and logic, since they raise problems peculiar to themselves. Perhaps the most familiar question in metaphysics is whether there are only material entities – materialism – or only mental entities, i.e., minds and their states – idealism – or both – dualism. Here ‘entity’ has its broadest sense: anything real. More specific questions of metaphysics concern the existence and nature of certain individuals – also called particulars – (e.g., God), or certain properties (e.g., are there properties that nothing exemplifies?) or relations (e.g., is there a relation of causation that is a necessary connection rather than a mere regular conjunction between events?). The nature of space and time is another important example of such a more specific topic. Are space and time peculiar individuals that ‘contain’ ordinary individuals, or are they just systems of relations between individual things, such as being (spatially) higher or (temporally) prior. Whatever the answer, space and time are what render a world out of the totality of entities that are parts of it. Since on any account of knowledge, our knowledge of the world is extremely limited, concerning both its spatial and temporal dimensions and its inner constitution, we must allow for an indefinite number of possible ways the world may be, might have been, or will be. And this thought gives rise to the idea of an indefinite number of possible worlds. This idea is useful in making vivid our understanding of the nature of necessary truth (a necessarily true proposition is one that is true in all possible worlds) and thus is commonly employed in modal logic. But the idea can also make possible worlds seem real, a highly controversial doctrine.
The notion of a spatiotemporal world is commonly that employed in discussions of the socalled issue of realism versus anti-realism, although this issue has also been raised with respect to universals, values, and numbers, which are not usually considered spatiotemporal. While there is no clear sense in asserting that nothing is real, there seems to be a clear sense in asserting that there is no spatiotemporal world, especially if it is added that there are minds and their ideas. This was Berkeley’s view. But contemporary philosophers who raise questions about the reality of the spatiotemporal world are not comfortable with Berkeleyan minds and ideas and usually just somewhat vaguely speak of ‘ourselves’ and our ‘representations.’ The latter are themselves often understood as material (states of our brains), a clearly inconsistent position for anyone denying the reality of the spatiotemporal world.
Usually, the contemporary anti-realist does not actually deny it but rather adopts a view resembling Kant’s transcendental idealism. Our only conception of the world, the anti-realist would argue, rests on our perceptual and conceptual faculties, including our language. But then what reason do we have to think that this conception is true, that it corresponds to the world as the world is in itself? Had our faculties and language been different, surely we would have had very different conceptions of the world. And very different conceptions of it are possible even in terms of our present faculties, as seems to be shown by the fact that very different scientific theories can be supported by exactly the same data. So far, we do not have anti-realism proper. But it is only a short step to it: if our conception of an independent spatiotemporal world is necessarily subjective, then we have no good reason for supposing that there is such a world, especially since it seems selfcontradictory to speak of a conception that is independent of our conceptual faculties. It is clear that this question, like almost all the questions of general metaphysics, is at least in part epistemological. Metaphysics can also be understood in a more definite sense, suggested by Aristotle’s notion (in his Metaphysics, the title of which was given by an early editor of his works, not by Aristotle himself) of ‘first philosophy,’ namely, the study of being qua being, i.e., of the most general and necessary characteristics that anything must have in order to count as a being, an entity (ens). Sometimes ‘ontology’ is used in this sense, but this is by no means common practice, ‘ontology’ being often used as a synonym of ‘metaphysics’. Examples of criteria (each of which is a major topic in metaphysics) that anything must meet in order to count as a being, an entity, are the following. (A) Every entity must be either an individual thing (e.g., Socrates and this book), or a property (e.g., Socrates’ color and the shape of this book), or a relation (e.g., marriage and the distance between two cities), or an event (e.g., Socrates’ death), or a state of affairs (e.g., Socrates’ having died), or a set (e.g., the set of Greek philosophers). These kinds of entities are usually called categories, and metaphysics is very much concerned with the question whether these are the only categories, or whether there are others, or whether some of them are not ultimate because they are reducible to others (e.g., events to states of affairs, or individual things to temporal series of events). (B) The existence, or being, of a thing is what makes it an entity. (C) Whatever has identity and is distinct from everything else is an entity. (D) The nature of the ‘connection’ between an entity and its properties and relations is what makes it an entity. Every entity must have properties and perhaps must enter into relations with at least some other entities. (E) Every entity must be logically self-consistent. It is noteworthy that after announcing his project of first philosophy, Aristotle immediately embarked on a defense of the law of non-contradiction.
Concerning (A) we may ask (i) whether at least some individual things (particulars) are substances, in the Aristotelian sense, i.e., enduring through time and changes in their properties and relations, or whether all individual things are momentary. In that case, the individuals of common sense (e.g., this book) are really temporal series of momentary individuals, perhaps events such as the book’s being on a table at a specific instant. We may also ask (ii) whether any entity has essential properties, i.e., properties without which it would not exist, or whether all properties are accidental, in the sense that the entity could exist even if it lost the property in question. We may ask (iii) whether properties and relations are particulars or universals, e.g., whether the color of this page and the color of the next page, which (let us assume) are exactly alike, are two distinct entities, each with its separate spatial location, or whether they are identical and thus one entity that is exemplified by, perhaps even located in, the two pages.
Concerning (B), we may ask whether existence is itself a property. If it is, how is it to be understood, and if it is not, how are we to understand ‘x exists’ and ‘x does not exist’, which seem crucial to everyday and scientific discourse, just as the thoughts they express seem crucial to everyday and scientific thinking? Should we countenance, as Meinong did, objects having no existence, e.g. golden mountains, even though we can talk and think about them? We can talk and think about a golden mountain and even claim that it is true that the mountain is golden, while knowing all along that what we are thinking and talking about does not exist. If we do not construe non-existent objects as something, then we are committed to the somewhat startling view that everything exists.
Concerning (C) we may ask how to construe informative identity statements, such as, to use Frege’s example, ‘The Evening Star