During the past twenty years or so, I have been working on ontological questions. What are universals, laws of nature, dispositions and powers, possibilities and necessities, classes, numbers? The present essay tries to bring all these topics together in a unified metaphysical scheme, an ontology. As a result, there is a certain amount of recapitulation of earlier writing. But putting the pieces together turned out to be quite difficult. A good deal of further work was necessary. Many mistakes, as I now think of them, had to be corrected. So what follows is not a mere sum of past thinking.

It has proved very hard to decide upon an order of exposition. Exposition must be serial, but I have continually felt hampered by this. It has often seemed that I could not discuss A without first having discussed B, but that to reverse the order would produce an equal and opposite problem. As a result, I do not know whether the chapters are in the best order. In particular, some readers may find it convenient to read the central chapter 8 at the beginning, or perhaps immediately after chapter 1, and then go back. Again, chapter 5, on Powers, is put in early because it is part of the theory of properties, and properties are discussed in chapters 3 and 4. But the topic of Powers is very closely linked with the discussion of Cause, and of Law, discussed in chapters 14 to 16. Chapters 5 plus 14 to 16 are a natural package.