Full Idea
I suggest 'tenacity of origin' rather than 'necessity of origin'. ..The most that we need is that Caesar's having something similar to his actual origin in certain respects (e.g. his actual parents) is normally kept fixed in counterfactual speculation.
Gist of Idea
Origin is not a necessity, it is just 'tenacious'; we keep it fixed in counterfactual discussions
Source
Penelope Mackie (How Things Might Have Been [2006], 6.9)
Book Reference
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.116
A Reaction
I find necessity or essentially of origin very unconvincing, so I rather like this. Origin is just a particularly stable way to establish our reference to something. An elusive spy may have little more than date and place of birth to fix them.