Full Idea
The familiar complaint against Lewis is that if his worlds existed, they'd be part of actuality.
Gist of Idea
If possible worlds really exist, then they are part of actuality
Source
comment on David Lewis (On the Plurality of Worlds [1986]) by Theodore Sider - Writing the Book of the World 11.5
Book Reference
Sider,Theodore: 'Writing the Book of the World' [OUP 2011], p.248
A Reaction
Sider presents that as rather superficial, but it sounds a pretty good objection to me. Lewis would note that only our world has the indexical features which he says pick out actuality. Real possible worlds might lack indexical features?