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Full Idea
One common argument to the conclusion that the Principle of the Identity of Indiscernibles is false is that it is not necessarily true.
Gist of Idea
The Identity of Indiscernibles is false, because it is not necessarily true
Source
Cynthia Macdonald (Varieties of Things [2005], Ch.2 n32)
Book Ref
Macdonald,Cynthia: 'Varieties of Things' [Blackwell 2005], p.75
A Reaction
This sounds like a good argument. If you test the Principle with an example ('this butler is the murderer') then total identity does not seem to necessitate identity, though it strongly implies it (the butler may have a twin etc).