Full Idea
This powerful puzzle (known to the Stoics, introduced by Geach, popularised by Wiggins) has a cat Tibbles and a proper part Tib, which is all of Tibbles except the tail. If Tibbles loses her tail, the two were distinct, but they now coincide.
Clarification
A 'proper' part is a part which is less than the whole
Gist of Idea
If Tib is all of Tibbles bar her tail, when Tibbles loses her tail, two different things become one
Source
Theodore Sider (Four Dimensionalism [2001], 5.1)
Book Reference
Sider,Theodore: 'Four Dimensionalism' [OUP 2003], p.142
A Reaction
[compressed] Compare a few people leave a football ground, and what was a large part of the crowd becomes the whole of the crowd. Which suggests that there is no problem if cats are like crowds. But we don't like that view of cats.