Of Thick Trees and Morality: An Analogy.

Envision in your mind an enormous tree containing many limbs, which branch off into smaller branches, which branch off into twigs, which sprout leaves. The branches and foliage are so thick that you can't see the  trunk of the tree at all.

Alexandre Calame, Landscape with Oak (1859)

Although you can't see the trunk, you can see the leaves. And you can see that they depend (for their existence and for their sustenance) upon the twigs, and that the twigs depend in turn upon the branches, which depend upon the limbs. From this, it seems to me that there are three basic conclusions you could draw: (a) the limbs are ultimately dependent upon something self-sustaining to sustain them; (b) the limbs are suspended in midair, relying upon nothing else; or (c) the limbs are dependent upon an infinite regress of other limbs, branches, etc., and that there's no bottom to this series.

Of these three, (b) seems to just wave away the phenomena, treating levitating branches as a brute fact, and (c) violates the rules of logic.So it seems to me that the best explanation would be to conclude to the existence of a trunk, or something like it. And this is so even though the trunk isn't directly observed. You can demonstrate its existence from its effects.

This is roughly analogous to the situation with the moral law. We see certain things that are morally true because of other, more fundamental truths; these truths are grounded in yet more fundamental truths, etc. Either this chain terminates in maximal truth (which we call God), or it arbitrarily stops, or it regresses infinitely.

This also works as an analogy to teleology: all moral actions are for the sake of some purpose (and "end"), or our action is pointless. But we can drive down deeper: we pursue this end for the sake of some further purpose, or it's a pointless end to pursue. And this goes on, until we either (a) arrive at a single final end; (b) arbitrarily stop [in which case, the whole chain is pointless], or (c) infinitely regress [which is logically impossible].

If (a) is true, we're grounding morality in a single final end, which we call God. If (b) is true, then morality is ultimately pointless.