Full Idea
Wiggins points out that identity is an atemporal relation whereas composition, like parthood, holds only relative to times.
Gist of Idea
Identity is an atemporal relation, but composition is relative to times
Source
report of David Wiggins (Sameness and Substance Renewed [2001]) by Theodore Sider - Four Dimensionalism 5.3
Book Reference
Sider,Theodore: 'Four Dimensionalism' [OUP 2003], p.161
A Reaction
If David Cameron is identical to the Prime Minister, that doesn't seem to be atemporal. If x=7 in this problem, I can change x to something else in the next problem. x had better not be equal to 7 and to 9.