This book began when I resolved to take a sabbatical leave in 1971–2. My proposed research consisted of three limited projects, as I thought: one on Aristotle's biology and metaphysics, one on Leibniz's monadology, and one on Frege's philosophy of language. Because I had written previously on the second and third of these topics, I decided (with a logic that seemed persuasive at the time but now escapes me) to work first on the Aristotle.

That was in the summer of 1971. I am writing these final words in the summer of 1986. Leibniz and Frege are still in my file drawer.

There have to be many reasons why a project should prove so intractable, besides the incapacities of its author, and there is little ground to expect the author to be aware of all of them. The main reason that I myself know about is the size and complexity of the subject, which I adequately realized only gradually. I have felt like a builder who contracted to construct a modest country church from a plan supplied by the vestry, but who finds that as each phase of the construction is supposedly completed, the plan has mysteriously become that of a larger and more elaborate building, and eventually that of a huge and pretentious cathedral – one which he would never have agreed to build at the start, but which he must now try to finish in view of the efforts already invested. (That is a personal and subjective simile; a different constructional image better suited to the subject-matter is pursued in §0.)