Full Idea
Stalnaker suggests talking 'ways things might have been' as sui generis elements of our ontology - actual abstract entities in their own right, not to be reduced to more familiar items.
Gist of Idea
We can take 'ways things might have been' as irreducible elements in our ontology
Source
report of Robert C. Stalnaker (Possible Worlds [1976]) by William Lycan - The Trouble with Possible Worlds 09
Book Reference
'The Possible and the Actual', ed/tr. Loux,Michael J. [Cornell 1979], p.303
A Reaction
This seems to rest on an ontology of 'states of affairs', favoured by Armstrong, and implied in the Tractatus. How big is a state of affairs? How manys states of affairs can be co-present?