Model 5

The omission of a "god agent" in no way affects this analysis.

Supposing there are two external standards, we ask the question "which external standard is the best, i.e. most good" or, alternately, "which of these standards ought to be used"?

We can arbitrarily state that the first standard is best, in which case the second standard disappears.
We can arbitrarily state that the second standard is best, in which case the first standard disappears.
We can recognize that a third moral standard is needed to compare against the first two. But if this standard exists, it has to be better than the two it is measuring, in which case it becomes the external standard.

Therefore, if an external moral standard exists, there must be at most one.

Next, does an external standard exist?