Full Idea
If it is in the nature of a possible object to be abstract, this is presumably a property it has in any possible circumstance in which it is actual. If it is actual it is also concrete. So the property of being abstract and concrete are not incompatible.
Gist of Idea
Possible objects are abstract; actual concrete objects are possible; so abstract/concrete are compatible
Source
Kit Fine (Intro to 'Modality and Tense' [2005], p.14)
Book Reference
Fine,Kit: 'Modality and Tense' [OUP 2005], p.14
A Reaction
A rather startling and powerful idea. What of the definition of an abstract object as one which is not in space-time, and lacks causal powers? Could it be that abstraction is a projection of our minds, onto concepts or objects?