Full Idea
Leibniz's Law, which a defender of relative identity might opt to reject, is so fundamental to the notion of identity that rejecting it amounts to changing the subject.
Clarification
The Law says two truly identical things must have the same properties
Gist of Idea
Leibniz's Law is so fundamental that it almost defines the concept of identity
Source
Colin McGinn (Logical Properties [2000], Ch.1 n8)
Book Reference
McGinn,Colin: 'Logical Properties' [OUP 2003], p.5
A Reaction
The Law here is the 'indiscernibility of identicals'. I agree with McGinn, and anyone who loses their grip on this notion of identity strikes me as losing all grip on reality, and threatening their own sanity (well, call it their 'philosophical sanity').