Sider’s (2001) modification of the Lewisean argument from vagueness for unrestricted mereological composition is advertised as having the advantage over the original that the assumption of the semantic determinacy of ‘part of’ (its lack of multiple eligible precisifications) is not required. This is not so; without this assumption the crucial step in Sider’s defence of his most contentious premiss, (P3), is one no defender of the linguistic theory of vagueness is obliged to take. Since the aim of the argument is (...) precisely to show that a defender of the linguistic theory of vagueness must accept unrestricted mereological composition the argument has no advantage over Lewis’s. (shrink)