Full Idea
The traits to suppose preserved in a counterfactual depend on sympathy for the fabulist's purpose. Compare 'If Caesar were in command, he would use the atom bomb', and 'If Caesar were in command, he would use catapults'.
Gist of Idea
What stays the same in assessing a counterfactual antecedent depends on context
Source
Willard Quine (Word and Object [1960], §46)
Book Reference
Quine,Willard: 'Word and Object' [MIT 1969], p.222
A Reaction
This seems to be an important example for the Lewis approach, since you are asked to consider the 'nearest' possible world, but that will depend on context.