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Full Idea
Lewis says that if not every class has a fusion then there must be a restriction on composition. The only plausible restrictions would be vague ones, which is impossible, because then whether composition occurs would be vague. So every class has a fusion.
Gist of Idea
There are no restrictions on composition, because they would be vague, and composition can't be vague
Source
report of David Lewis (On the Plurality of Worlds [1986], p.212-3) by Theodore Sider - Four Dimensionalism 9.1
Book Ref
Sider,Theodore: 'Four Dimensionalism' [OUP 2003], p.121
A Reaction
This is Lewis's key argument in favour of unrestricted composition, his Vagueness Argument. Why can't composition be vague? If you gradually reassemble a broken mirror, at what point does the mirror acquire its unity?