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What Is Metaphysics?

The great Greek philosopher Plato wrote (in the dialogue Theaetetus) that philosophy "begins in wonder," a phrase repeated by his student Aristotle in his Metaphysics. This is especially true of that branch of philosophy that we, echoing the title of Aristotle's book, call "metaphysics." In metaphysics we puzzle and wonder about what exists and what existing things are like, in their most fundamental features and interrelationships.

1.1 The Subject of Metaphysics

The first part of metaphysics is known as "ontology," the study of what there is. In ontology we attempt to give, in broad outlines, an inventory of reality. Are there particular things, such as cabbages, kings, quarks, and galaxies? How many such things are there? One? Many? Infinitely many?

Are there properties, ways things are? For example, is there, in addition to all the individual horses, the property of being a horse (equinity)? If so, how many such properties are there? Is there a property for every common noun and every adjective? A property of being red, of being ugly, of sleeping? Do some of these properties exist as separate universals? That is, is one and the same property somehow shared by everything that has that property? If so, do each of these universals inhere within many particular things, or do they in some other way explain the similarities and common characteristics of those many particular things? And are there relations, like that of being more massive than or being the same color as, that hold between or among two or more things?

Are there things that could be called "facts" or "states of affairs," such as the fact that water molecules contain hydrogen or the state of affairs of all native mammals in Australia's being marsupials? Are there negative facts, such as the fact that there is no plant life on the sun? If there are facts, are they the things that, by simply existing, are responsible for making certain beliefs and statements true? Is truth itself a property, and if so, of what things? Do facts contain both particular things and universal properties? Are there merely possible facts, and if so, what are they like? What is the fundamental difference between merely possible facts and the actual ones?

Other parts of metaphysics constitute the study of the fundamental structure of reality as a whole. How do things fit together to make a world? Plato describes this task of philosophy "carving nature at the joints," comparing the metaphysics to a skillful and knowledgeable act of dissection. Here are four relations that seem to be among the fundamental relations of this worldly structure: the relation between things and their properties, between wholes and their parts, between causes and effects, and between things related to each other in space and in time. We will examine all of these foundational relations in some detail.

1.2 The Methods of Metaphysics

Since metaphysicians study reality in its most fundamental and general aspects, in doing it we must marshal as much evidence about the world as we possibly can. All of our knowledge of the world, whether innate or acquired through ordinary life or through specialized sciences, contributes data to the metaphysical theorist. So, too, do hunches and intuitions of the truth, when more secure knowledge is unavailable. The method of the metaphysician is a mixture of the testimony of pure reason, that which is prior to and independent of experience (the a priori), and the testimony of experience itself (the a posteriori), in all its breadth and variety. Metaphysics is in this way like most other sciences. (We use the word "science" here in a broad sense, as a label of any systematic field of knowledge.)

What exactly the methods of metaphysics should be is one of the most hotly disputed topics among metaphysicians. In addition, some critics have disputed the very right of metaphysics to exist as a separate science. They argue that we best study the fundamental nature of reality through some more specialized discipline, such as physics, history, psychology, or linguistics. However, such thinkers do not thereby avoid doing metaphysics - instead, they do metaphysics in a particular way, with an especially truncated set of data and methods. In this book, we will attempt to be relatively broad and inclusive in our survey of metaphysical methods, including input from all of the natural and human sciences, as well as from that source of knowledge that we call "common sense," anchored in the common experience of humanity.

Another methodological issue that divides metaphysicians is the question of the role that speculation or invention should play. Some metaphysicians seek simple, elegant, and unifying theories, including the postulation of novel entities and properties, while others seek metaphysics as a kind of grammar of ordinary human thought and experience, as merely making explicit what every adult human being knows. On either view, metaphysics can reach results that are surprising, even revolutionary, as the example of mathematics demonstrates: geometry and number theory are able to derive many new and useful results, starting from nothing but a few commonplaces about numbers or space.

1.3 The Waxing and Waning of Metaphysics

Metaphysics is the oldest branch of philosophy, already underway in the speculations of ancient Greek-speaking thinkers, including Thales, Heraclitus, and Parmenides, in Greece, Turkey, and southern Italy in the 600s and 500s bc. Metaphysics continued to be central to the work of Plato and Aristotle, as well as to the "materialism" of Democritus and Empedocles, who sought to answer all of the metaphysical questions in terms of the fundamental material components of things. During the Hellenistic period (between the conquests of Alexander the Great and the rise of Rome), the central focus of philosophy shifted from metaphysics to the theory and critique of knowledge (the branch of philosophy known as "epistemology"), although both the new philosophical schools of the Stoics and Epicureans and the successors of Plato and Aristotle continued in substantial metaphysical investigations.

Metaphysics regained its predominance in late antiquity and throughout the medieval periods, thanks to the pre-eminence within Western philosophy of Platonists and Aristotelians. A synthesis of the two traditions, known as "Scholastic" philosophy, provided a common framework of terminology, questions, and methods among Christians, Jews, and Moslems for over a thousand years. In the later Middle Ages (after 1300 ad), there was a gradual turn toward the study of language and toward epistemological concerns. During the Renaissance, scholars sought to return to ancient sources, including Plato. At the same time, these scholars recovered writings of some of the ancient materialists and Atomists, such as Democritus and Lucretius, which then began to influence the course of Western philosophy.

The scientific revolution of the sixteenth century brought with it a revolt against Aristotle and Scholasticism. Metaphysical thought fragmented into several distinct streams. Some, including the German seventeenth-century philosopher Gottfried Leibniz, continued important features of the Scholastic tradition. Others turned instead to a form of Atomism about the created world, reviving Democritus's idea that the material world is ultimately composed of indivisible "corpuscles." (We say "created" world because these thinkers believed in the existence of God.) Such Atomism had been rejected by Aristotle and his Scholastic followers in favor of a view in which all of matter is infinitely divisible. The French philosopher René Descartes introduced a new form of dualism, which divided reality into two domains, one purely quantitative and material (the physical), and the other qualitative and subjective (the mental). Still others moved all the way to Idealism, the view that all of reality, including the natural world, is fundamentally mental or spiritual. In fact, Idealism of one kind or another dominated European and American philosophy in the nineteenth century.

Descartes altered the course of Western philosophy for 200 years by introducing an overriding concern, amounting almost to an obsession, with attaining certainty. Descartes held that it was the responsibility of philosophers to provide a watertight answer to the challenge of the skeptic, who insists upon doubting every belief that can possibly be doubted. The Cartesian philosopher seeks to build an absolutely secure and indubitable set of foundations for all of our scientific and common sense beliefs. This quest for certainty necessitated a turn inward, relying on Descartes' famous cogito argument: I think, therefore I am ("cogito ergo sum" in Latin). The guiding idea was that introspection of one's own subjective thoughts and feelings was immune to skeptical challenge. One might be wrong about the past or about the physical facts, but one cannot be wrong about the present contents of one's own thoughts and experiences. It was this broad agreement about the subjective or "phenomenological" method that gave the advantage to various forms of Idealism in the mid- to late nineteenth centuries.

1.4 Modern Challenges to Metaphysics

With the loss of the Scholastic framework, the rise of the success...