Full Idea
I take the 'overlap requirement' for Julius Caesar to be that, when considering how he might have been different, you have to take him as he actually was at some time in his existence, and consider possibilities consistent with that.
Gist of Idea
Possibilities for Caesar must be based on some phase of the real Caesar
Source
Penelope Mackie (How Things Might Have Been [2006], 6.5)
Book Reference
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.108
A Reaction
This is quite a large claim (larger than Mackie thinks?), as it seems equally applicable to properties, states of affairs and propositions, as well as to individuals. Possibility that has no contact at all with actuality is beyond our comprehension.