  Whether non-naturalism really is less vulnerable to the challenge is not clear, however. On the one hand, the assumption that moral properties are sui generis may help realists to defend the exceptionalist view that the reference of moral terms is determined in a special way (at least along with terms in other domains that deal with non-natural properties). On the other hand, explaining how our moral terms have come to refer to such properties may be extra difficult, especially given the further assumption that they are causally inert (the issue is discussed in Suikkanen 2017). It should also be noted that the soundness of at least the charity-based versions of the challenge seems unaffected by what view one takes on the nature of moral properties. On those versions, systematic differences regarding the application of moral terms threaten to undermine co-reference regardless of whether the candidate properties to which those terms refer are taken to be non-natural or not.