Full Idea
Wiggins argues that Geach's Relative Identity is incompatible with the formal properties of identity, which include, besides transitivity, symmetry and reflexivity, the complete community of properties defined by the Indiscernibility of Identicals.
Clarification
The Indiscernibility of Identicals is often called Leibniz's Law
Gist of Idea
Relative Identity is incompatible with the Indiscernibility of Identicals
Source
report of David Wiggins (Sameness and Substance Renewed [2001]) by Peter F. Strawson - Review of 'Sameness and Substance' p.603
Book Reference
-: 'Mind' [-], p.603
A Reaction
The tricky part is that Wiggins then goes on to say that identity depends on sortals, which sounds very close to the Geach view. I find disentangling them tricky. See Idea 14363 for a helpful comment from Strawson.
Related Idea
Idea 14363 Identity a=b is only possible with some concept to give persistence and existence conditions [Wiggins, by Strawson,P]