Full Idea
The abstract/concrete distinction is, roughly, between those sortals whose grounding relations can hold between abstract things which are spatially but not temporally separated, those concrete things whose grounding relations cannot so hold.
Gist of Idea
Being abstract is based on a relation between things which are spatially separated
Source
Bob Hale (Abstract Objects [1987], Ch.3.III)
Book Reference
Hale,Bob: 'Abstract Objects' [Blackwell 1987], p.59
A Reaction
Thus being a father is based on 'begat', which does not involve spatial separation, and so is concrete. The relation is one of equivalence.