Where to begin?

There are a variety of possible starting-points for discussing David Lewis's work. I suspect the most common way in is for people to start with his work on a particular topic they are interested in – causation, the mind, convention, properties and relations, or whatever – and then gradually come to see how his views and arguments in one area connect with his views and arguments in another, until some greater or smaller piece of an entire system emerges. So where should a general introduction start? I have decided to start with Lewis's metaphysics, perhaps the most philosophically influential part of his work, and a part of the system that can be seen as being at the base of an entire worldview. Lewis's views in other areas do stand or fall to an extent independently of his metaphysical views. Whereas Lewis does have a system, it is not a system where every part presupposes the others, but rather a system where views in many different areas are developed and argued for in their own terms, although there are many points of contact between different doctrines. Looking at his metaphysical views first will establish a framework for locating his other doctrines.

So if I am to start with metaphysics, what is metaphysics? Lewis, who did so much to shape the debates in metaphysics in English-speaking professional philosophy, never, to my knowledge, attempts a definition of the subject.