Full Idea
If someone is in pain at t1 and not at t2, we might restrict Leibniz's Law so as not to apply to 'timebound' properties, ..but this is deeply unsatisfying, ...and forfeits one's claim to be discussing identity. The demands of identity are high.
Gist of Idea
If you say Leibniz's Law doesn't apply to 'timebound' properties, you are no longer discussing identity
Source
Theodore Sider (Four Dimensionalism [2001], 5.5)
Book Reference
Sider,Theodore: 'Four Dimensionalism' [OUP 2003], p.166
A Reaction
[on Myro 1986] Sider's response is unsatisfying. It means a thing loses its identity (with itself?) if it has even a tiny fluctuating in its properties. Quantum changes then destroy all notions of identity. English-speakers don't use 'identity' like that.