I haven't been following the grounding literature, so this may be old hat, in which case I will be grateful for references.
The following seems pretty plausible:
- p is fundamental if and only if p is ungrounded.
Let's suppose that <I ought to respect other persons> is a fundamental moral truth. Call this truth R. But now I validly promise to respect other persons. Then R comes to be grounded in <I ought to keep my promises and I promised to respect other persons>. If (1) is true, then R continues to be true but ceases to be fundamental. That doesn't sound right. It seems to me that if R is ever a fundamental moral truth, then it is always a fundamental moral truth. After I have promised to respect other persons, R gained a ground but lost nothing of its fundamentality.
Maybe I can motivate my intuition a little more. It seems that R has a relevantly different status from the status had by S, the proposition <I ought to come to your house for dinner every night>, after I promise you to come to your house for dinner every night. Each of R and S is grounded by a proposition about promises, but intuitively the fundamentality-and-grounding statuses of R and S are different. A sign (but only a sign--we want to avoid the conditional fallacy) of the difference is that R would still be true were the proposition about promises false. Another sign of the difference is that <I ought to respect you> is overdeterminingly grounded in <I ought to respect all persons> and <I promised to respect all persons and I ought to keep my promises>, while it is false that <I ought to come for dinner tomorrow night> is overdeterminingly grounded in <I ought to come for dinner every night> and <I promised to come for dinner every night and I ought to keep my promises>. The latter is not a case of overdetermination.
The above example is controversial, and I can't think of any noncontroversial ones. But it seems plausible that we should be open to phenomena like the above. Such prima facie possibilities suggest to me that ungroundedness is a negative property, while fundamentality is something positive. Normally, fundamental truths are also ungrounded. But they don't lose their fundamentality if in some world they happen to be grounded as well.
A somewhat tempting way to keep the above intuition while maintaining the idea that fundamentality is to drop the irreflexivity of grounding and say that:
- p is fundamental if and only if p grounds p.