For what reason did Dharmakirti argue that absences are conceptual constructions?

For what reason did Dharmakirti argue that absences are conceptual constructions? I wondered if it was because real absences would have svabhava, would be essences, because they do not change in time? I can't find anything to cite now, but I believe he argued this. I mean everyday absences, such as a cow from a field, rather than anything technical about apoha, which I am not that familiar with.