Ontology is the study of “being,” i.e. what it means for something to “be” or “exist.” I have discussed on other occasions the ontology of time and the ontology of logic and mathematics (among other things, in numerous places). A couple weeks ago for a Christian youth group (Stand to Reason) I was asked to discuss the ontology of moral facts, expanding on questions that remained after last year. A crucial section of my Sense and Goodness without God on all this is III.5 (pp. 119-34), esp. III.5.4-5 (pp. 124-34).